View Full Version : Bolt Carrier exhaust vent size variations
BufordTJustice
01-01-15, 12:17
Prepare to be disappointed because i have no pictures. However, I was thinking about the size disparity in the exhaust gas ports of various bolt carriers.
I have noticed that BCM, Colt, and LMT all tend to have noticeably larger exhaust gas vent ports in their bolt carriers as compared to many others I've seen.
I know that the exhaust gas ports only play a part in the function of the rifle system, but this occurred to me recently (call it an epiphany). I'll do my best to gather some pictures in a few minutes.
Some notes: This is not a **** measuring contest, comparing the above listed manufacturers to others and holding them out as supreme. I want to discuss WHY the ports of most other carriers are smaller....whether good or bad. This is to be a technical discussion, not brand war.
BufordTJustice
01-01-15, 13:36
Photos.
Courtesy of Trident several years ago:
http://i54.tinypic.com/2cqmgp0.jpg
Thread link:
https://www.m4carbine.net/showthread.php?79744-BCM-bolt-carrier-group&p=981243#post981243
You can see that, even among the Colt/BCM/LMT, the LMT has a slightly smaller port size. Now get ready for a YM, which has some really small ports and is exemplary of "small" ports that I have seen:
http://combatrifle.net/news/young_mfg_bcg.jpg
Another pic I grabbed off of Google images (Top to bottom: YM, YM, DPMS, CMT):
http://i149.photobucket.com/albums/s70/lilbi0tch/IMG_4028_edited.jpg~original
The LMT Enhanced carrier has a third hole as I'm sure you know. While I'm sure function is the primary goal, I see larger or more vent holes as an aid in cleaning as it allows for more gas residue to blow out as opposed to being trapped inside and deposited there.
BufordTJustice
01-01-15, 14:38
The LMT Enhanced carrier has a third hole as I'm sure you know. While I'm sure function is the primary goal, I see larger or more vent holes as an aid in cleaning as it allows for more gas residue to blow out as opposed to being trapped inside and deposited there.
I run a gen-4 LMT e-carrier ad you are correct. The porting also affects the back half of the pressure curve of the bolt carrier acceleration from the gas pulse. It's a relatively minor effect compared to gas port size, buffer weight/reciprocating mass, etc. The Army ran some tests way back when where they had a carrier with no ports and, while it did increase BCG rearward velocity, the guns (20" A1's) still functioned. Just not as well as with the mil spec ports we see as std today.
I run a gen-4 LMT e-carrier ad you are correct. The porting also affects the back half of the pressure curve of the bolt carrier acceleration from the gas pulse. It's a relatively minor effect compared to gas port size, buffer weight/reciprocating mass, etc. The Army ran some tests way back when where they had a carrier with no ports and, while it did increase BCG rearward velocity, the guns (20" A1's) still functioned. Just not as well as with the mil spec ports we see as std today.
Take a look at the admission port on your enhanced carrier. That's an interesting subject on its own.
BufordTJustice
01-01-15, 17:58
Take a look at the admission port on your enhanced carrier. That's an interesting subject on its own.
Agreed. The third primary port AND the fourth smaller port in the cut-out area....fascinating stuff indeed.
Collin,
I don't believe the hole size makes that much difference.
Most of the pressure event is over before the vents come into play.
I measured two carriers that were handy.
The YM carrier holes were .093"
The mil-spec carrier holes were .109"
Either one seems fine.
BufordTJustice
01-01-15, 22:01
Collin,
I don't believe the hole size makes that much difference.
Most of the pressure event is over before the vents come into play.
I measured two carriers that were handy.
The YM carrier holes were .093"
The mil-spec carrier holes were .109"
Either one seems fine.
Gotcha. I know that the holes didn't even come into play until pressure was already dropping.
Could it be that smaller holes might give a slight margin with lower pressure ammo, or is the difference too small to matter?
sinlessorrow
01-02-15, 01:01
Collin,
I don't believe the hole size makes that much difference.
Most of the pressure event is over before the vents come into play.
I measured two carriers that were handy.
The YM carrier holes were .093"
The mil-spec carrier holes were .109"
Either one seems fine.
He may be on to something here. There has been a lot of ECP's over the years that modified the TDP, this may be one of them.
sinlessorrow
01-02-15, 01:09
Gotcha. I know that the holes didn't even come into play until pressure was already dropping.
Could it be that smaller holes might give a slight margin with lower pressure ammo, or is the difference too small to matter?
Watch this video.
https://www.youtube.com/watch?v=xPwTGfhUulk
You really may be on to something.
On of our members here was talking to me about the idea of an experiment where the exhaust ports would be welded shut. ??
On a standard carrier, you can temporarily plug either 2 exhaust ports and/or or debris port by pressing in a piece of lead shot into it. In some cases, you pick up some carrier speed, but get a lot more fouling. I would tend to lean away from smaller ports in the standard location because of this.
On a standard carrier, you can temporarily plug either 2 exhaust ports and/or or debris port by pressing in a piece of lead shot into it. In some cases, you pick up some carrier speed, but get a lot more fouling. I would tend to lean away from smaller ports in the standard location because of this.
Yeah... makes sense. Seems like the ports may be there to spit out look carbon particles.
sinlessorrow
01-02-15, 09:07
Yeah... makes sense. Seems like the ports may be there to spit out look carbon particles.
Which makes me wonder why are there differences? From what I can Colt and BCM are the only 2using the large ports. My S&W MP4 carrier matches the LMT ones, which is smaller than Colts.
Not surprisingly, the Army did extensive testing on many aspects of the M16.
Some of this is documented in BRL 1548 from August 1971.
One variation tested was normal (.109") vent holes vs NO vent holes.
The no vent test resulted in approximately 2% increase in gas drive / bolt carrier momentum.
They even calculated the effect of moving the vents to dump pressure sooner in the recoil stroke.
Calculations show this has very little effect as well.
Not surprisingly, the Army did extensive testing on many aspects of the M16.
Some of this is documented in BRL 1548 from August 1971.
One variation tested was normal (.109") vent holes vs NO vent holes.
The no vent test resulted in approximately 2% increase in gas drive / bolt carrier momentum.
They even calculated the effect of moving the vents to dump pressure sooner in the recoil stroke.
Calculations show this has very little effect as well.
Very interesting, makes you wonder what testing was done internally by Colt/BCM/LMT to determine that it was beneficial to make their BCGs the way that they have. Perhaps it mattered not at all and was the whim of an engineer, but you would like to believe that these manufacturers chose their dimensions based upon a more solid analysis of the design.
When LMT added the small bleed hole on there enhanced carrier, I wasn't so convinced that the small hole could do much of anything. So, I grabbed a 20" adjustable gas upper and a 10.5" adjustable gas upper, ran the new carrier in both on regular rifle SBR lower. I set the gas to just lock back 100% on some 77gr match ammo I had. I then plugged the small vent hole. It didn't seem to effect the 20" much at all, ran the same with the bleed open. On the 10.5", I was able to close down on the gas more and still retain 100% lock back.
My best idea of a conclusion was that the rifle gas didn't pressurize the carrier as much as the carbine one, so less bled out of the small bleed hole compared to the carbine.
sinlessorrow
01-02-15, 09:37
Very interesting, makes you wonder what testing was done internally by Colt/BCM/LMT to determine that it was beneficial to make their BCGs the way that they have. Perhaps it mattered not at all and was the whim of an engineer, but you would like to believe that these manufacturers chose their dimensions based upon a more solid analysis of the design.
I would say BCM follows Colt when it comes to their carrier, and Colt probably had a reason.
Once my Colt BCG gets here I can measure them and get us a better difference in my 2 carriers.
The KMR upper I have has smaller exhaust ports on it than some others BCMs I've had in the past. Along the lines of Clint's post... I think this is completely negligible.
Not surprisingly, the Army did extensive testing on many aspects of the M16.
Some of this is documented in BRL 1548 from August 1971.
One variation tested was normal (.109") vent holes vs NO vent holes.
The no vent test resulted in approximately 2% increase in gas drive / bolt carrier momentum.
They even calculated the effect of moving the vents to dump pressure sooner in the recoil stroke.
Calculations show this has very little effect as well.
I have seen a few exhaust ports elongated to dump earlier in the cycle on a some over gassed rifles. I felt it was a poor fix for an excessively large gas port, but it did seem to tame them down a bit. So, I'm pretty curious as to why the calculations listed above would show otherwise?
sinlessorrow
01-02-15, 09:47
The KMR upper I have has smaller exhaust ports on it than some others BCMs I've had in the past. Along the lines of Clint's post... I think this is completely negligible.
We need an engineer stat.
The KMR upper I have has smaller exhaust ports on it than some others BCMs I've had in the past. Along the lines of Clint's post... I think this is completely negligible.
The small change in diameter could be negligible, but what benefit could it have?
When the key disengages the gas tube, there is fouling that blows both from the key and tube. I would tend to think that dumping as much out the exhaust port as possible is better than inside the upper?
The small change in diameter could be negligible, but what benefit could it have?
When the key disengages the gas tube, there is fouling that blows both from the key and tube. I would tend to think that dumping as much out the exhaust port as possible is better than inside the upper?
Following in the same thought above. The LMT Enhanced carrier has a different gas admission port in the carrier compared to the more common hole, it has twin orifices aimed around the bolt tail. Since restrictions work in both directions, I would think that the enhanced carrier would vent less gas out the key and more out the exhaust ports, maybe that's why a third one was added? The gas admission port on a conventional carrier becomes an exhaust port when the key disengages the gas tube.
Just food for thought.
I never truly understood everything behind the different number and size of the holes. But just to add more material for discussion, here is the Primary Weapons Systems DI (not piston) bolt carrier with just one big hole where there are normally two small ones. Also, I know these bolt carriers are significantly heavier (a little more than an ounce heavier) than a mil-spec carrier.
30793
[You need to click on the picture to see a larger image. Sorry, I don't know how to make this picture bigger without having to click on it. A moderator can feel free to edit my post/picture if they want.]
Joe Mamma
BufordTJustice
01-02-15, 12:52
At work, but this is recent.
LAV FIREClean Lube Test: http://youtu.be/S0OAsOCEJfQ
Without putting my foot in my mouth, I can say that having the exhaust ports a little late in the cycle is likely far more detrimental than having the ports a little too soon in it. I could see smaller ports venting enough quantity of gas given the extra time if it happens a little earlier.
If using a standard key and gas tube for a given system, it's a good idea to vent as much out the ports facing outwards than out the key.
Things can get changed up when altering the cam path. That's another interesting subject.
sinlessorrow
01-02-15, 22:32
Just checked and my MP4 BCG has vent ports that are .1" in size.
Following in the same thought above. The LMT Enhanced carrier has a different gas admission port in the carrier compared to the more common hole, it has twin orifices aimed around the bolt tail. Since restrictions work in both directions, I would think that the enhanced carrier would vent less gas out the key and more out the exhaust ports, maybe that's why a third one was added? The gas admission port on a conventional carrier becomes an exhaust port when the key disengages the gas tube.
Just food for thought.
That would explain why I can't pass a pipe cleaner all the way into the carrier when cleaning the Enhanced carrier key.
The gas tube has .650" of engagement with the key before the "seal" is broken.
The vents open after ~ .270" of carrier travel.
Extraction starts at .325" of carrier travel.
The gas key disengages .380" after the vents open and .325" after extraction starts.
From this, it seems like most of the venting takes place through the vents rather than the gas key.
When the key disengages the gas tube, there is fouling that blows both from the key and tube. I would tend to think that dumping as much out the exhaust port as possible is better than inside the upper?
BufordTJustice
01-03-15, 14:43
The gas tube has .650" of engagement with the key before the "seal" is broken.
The vents open after ~ .270" of carrier travel.
Extraction starts at .325" of carrier travel.
The gas key disengages .380" after the vents open and .325" after extraction starts.
From this, it seems like most of the venting takes place through the vents rather than the gas key.
So, Clint, could a buffer heavy enough to delay unlocking also result in a substantively cleaner receiver due to delaying the separation of the gas key and gas tube?
The gas tube has .650" of engagement with the key before the "seal" is broken.
The vents open after ~ .270" of carrier travel.
Extraction starts at .325" of carrier travel.
The gas key disengages .380" after the vents open and .325" after extraction starts.
From this, it seems like most of the venting takes place through the vents rather than the gas key.
I agree for the most part, the majority blows out the exhaust ports, but gas does flow out the key and gas tube when it disengages.
I would think so.
Anything that lowers carrier velocity will make extraction and gas tube separation happen later relative to the projectile leaving the muzzle.
This includes increasing operating mass, reducing gas drive and using a longer gas system.
Remember from the Best Suppressed SBR ever (https://www.m4carbine.net/showthread.php?33743-Best-Suppressed-SBR-Ever) thread, "suppressor small" gas ports virtually eliminates gassy face.
So, Clint, could a buffer heavy enough to delay unlocking also result in a substantively cleaner receiver due to delaying the separation of the gas key and gas tube?
BufordTJustice
01-03-15, 19:16
I would think so.
Anything that lowers carrier velocity will make extraction and gas tube separation happen later relative to the projectile leaving the muzzle.
This includes increasing operating mass, reducing gas drive and using a longer gas system.
Remember from the Best Suppressed SBR ever (https://www.m4carbine.net/showthread.php?33743-Best-Suppressed-SBR-Ever) thread, "suppressor small" gas ports virtually eliminates gassy face.
So, yet again, the A5 system could actually help a gun run cleaner if it were grossly under-buffered to begin with?
Interesting.
Now for a non-rhetorical question: how does an altered cam pin track come in to play with regard to the timing of bolt unlocking, excess gas in the receiver, etc. Tom touched on this earlier in the thread.
Off topic, but Clint and tom12.7, I am grateful you two grace this forum with your knowledge. It seems like every time i read one of your posts I feel smarter than before.
Another thing you can see on the video posted on post #10 is a small residual amount of gas venting out the gas tube hole in the upper receiver, which has the 4 vent holes offset from the main hole for the tube itself.
http://762precision.files.wordpress.com/2012/04/truing-the-upper-receiver.jpg
On the Dutch AR10's, the carrier key is much longer than the AR15. Jim Sullivan said that the AR15 needs a longer carrier key. A longer key would keep the gas relief area limited to the vents on the bolt carrier longer, so that the residual gas was vented out once the key clears the tube.
This whole discussion, in addition to others, makes me wonder how much of this internal gas expansion system and carbon management had to do with Stoner's choice of hard chroming of the BCG on the AR10, which subsequently was integral to the AR15 prototypes and first 2 production AR15's (Colt 601 and 602).
I just measured my Colt early 1960's hard chrome, slickside BCG, and the vent holes are at least .110", if not .120". The gun it's in is a 605 clone, which has a RLGS with a 15.5" barrel, with hardly any dwell time, and it still runs great on XM193F. It's never malf'd on me, and I have not opened the gas port from .094". I'm going to shoot it in extreme cold, which was one of the main killers of the 605, to see if the port needs opening like most people do with the retro 605 builds.
Leaveammoforme
01-04-15, 02:40
Just to add another thought to the process and I have zero references for this.
I have always been under the impression that the larger vents were to aid in liquid drainage.
Let's discuss a little about altering the cam path in the carrier. We can start really basic and proceed to more complex later.
Sullivan was mentioned a little earlier, he put together a project with others for multiple possible improvements in the platform. One of his improvements was a double cut in the cam way to increase carrier travel before the unlocking of the bolt begins. Basically keeping the same cam cut, and superimposing an identical cut offset to slightly increase the carrier stroke before the radial cut initiates cam pin/bolt rotation. Say we build a carrier that is otherwise 100% to the TDP, except this extended cut. Use this modified carrier, with all other TDP parts for their intended roles. 20" rifles, 14.5" and 10.5" carbines as a baseline that can be expanded somewhat later.
How do you suppose the characteristics would be in general?
During the action cycling sequence, the BCG would be able to travel further with only action resistance before initiating unlocking. That increased distance takes time and allows more pressure drop in the chamber, easing unlocking, and reducing stress on the bolt. What else is going on though? The initial travel with only action resistance allows the carrier to accelerate to a higher speed before unlocking. The added time decreases chamber pressure during unlocking, thus requiring less work to operate. With less work, the carrier speed isn't slowed as much as it would if it required the original work, again additional carrier velocity. So in general, during cycling, this would be a faster carrier.
How would this effect a 20" rifle? Out of the 3 rifle samples, the 20" should see the least gain in carrier velocity. It already unlocks at a reduced pressure than the carbines, plus has increased reciprocating mass over them. This would help keep the velocity increase to a minimum.
What about the 14.5" carbine? The original BCG was intended for 20" inch guns. When used in a 14.5", unlocking occurs during a higher chamber pressure than the 20". The system is gassed to give it more energy to cycle the action, as the the unlocking requires more work to be done. How does this effect the 14.5" with the altered carrier? There would be a larger jump in carrier velocity compared to the 20", as the system is gassed harder and has less reciprocating mass.
What about the 10.5" carbine? This would be more of the same as the 14.5", just even more of a carrier velocity than the others.
So, what could be done to reduce the speeds in these instances? Dedicated gas metering would probably be a good direction to look into, if it takes less work, you don't need as much gas obviously. There's other benefits of needing less gas, fouling being one of them. Another option on the carbines is to make their action system rifle like to keep the speed in check (think A5). Really a combination of the two is probably the best solution for carbines.
Let's chew on this for a little while.
sinlessorrow
01-04-15, 09:55
Let's discuss a little about altering the cam path in the carrier. We can start really basic and proceed to more complex later.
Sullivan was mentioned a little earlier, he put together a project with others for multiple possible improvements in the platform. One of his improvements was a double cut in the cam way to increase carrier travel before the unlocking of the bolt begins. Basically keeping the same cam cut, and superimposing an identical cut offset to slightly increase the carrier stroke before the radial cut initiates cam pin/bolt rotation. Say we build a carrier that is otherwise 100% to the TDP, except this extended cut. Use this modified carrier, with all other TDP parts for their intended roles. 20" rifles, 14.5" and 10.5" carbines as a baseline that can be expanded somewhat later.
How do you suppose the characteristics would be in general?
During the action cycling sequence, the BCG would be able to travel further with only action resistance before initiating unlocking. That increased distance takes time and allows more pressure drop in the chamber, easing unlocking, and reducing stress on the bolt. What else is going on though? The initial travel with only action resistance allows the carrier to accelerate to a higher speed before unlocking. The added time decreases chamber pressure during unlocking, thus requiring less work to operate. With less work, the carrier speed isn't slowed as much as it would if it required the original work, again additional carrier velocity. So in general, during cycling, this would be a faster carrier.
How would this effect a 20" rifle? Out of the 3 rifle samples, the 20" should see the least gain in carrier velocity. It already unlocks at a reduced pressure than the carbines, plus has increased reciprocating mass over them. This would help keep the velocity increase to a minimum.
What about the 14.5" carbine? The original BCG was intended for 20" inch guns. When used in a 14.5", unlocking occurs during a higher chamber pressure than the 20". The system is gassed to give it more energy to cycle the action, as the the unlocking requires more work to be done. How does this effect the 14.5" with the altered carrier? There would be a larger jump in carrier velocity compared to the 20", as the system is gassed harder and has less reciprocating mass.
What about the 10.5" carbine? This would be more of the same as the 14.5", just even more of a carrier velocity than the others.
So, what could be done to reduce the speeds in these instances? Dedicated gas metering would probably be a good direction to look into, if it takes less work, you don't need as much gas obviously. There's other benefits of needing less gas, fouling being one of them. Another option on the carbines is to make their action system rifle like to keep the speed in check (think A5). Really a combination of the two is probably the best solution for carbines.
Let's chew on this for a little while.
The only issue with increase the cam pin travel is you then need to modify the position of the extractor pin, as when fully unlocked the pin will not be retained by the carrier. Or you need to make a new carrier that is extended in the front to retain the cam pin. The other option is the KAC route of adding a retaining spring into the extractor.
Either way you need to change the whole BCG to make a change in cam pin travel possible
The only issue with increase the cam pin travel is you then need to modify the position of the extractor pin, as when fully unlocked the pin will not be retained by the carrier. Or you need to make a new carrier that is extended in the front to retain the cam pin. The other option is the KAC route of adding a retaining spring into the extractor.
Either way you need to change the whole BCG to make a change in cam pin travel possible
I agree, I omitted positive extractor retention to keep it simple. I was planning on mentioning the KAC E3 extractor later, and the LMT enhanced carrier extended lip if we moved on to talk about the LMT one specifically.
Interesting.
I never knew the Dutch AR10s had longer gas keys.
They also have 3 vent holes.
Found a pic.
http://i220.photobucket.com/albums/dd74/eyegun/AR-10/IMG_2598.jpg
http://i220.photobucket.com/albums/dd74/eyegun/AR-10/IMG_2597.jpg
On the Dutch AR10's, the carrier key is much longer than the AR15. Jim Sullivan said that the AR15 needs a longer carrier key. A longer key would keep the gas relief area limited to the vents on the bolt carrier longer, so that the residual gas was vented out once the key clears the tube.
This whole discussion, in addition to others, makes me wonder how much of this internal gas expansion system and carbon management had to do with Stoner's choice of hard chroming of the BCG on the AR10, which subsequently was integral to the AR15 prototypes and first 2 production AR15's (Colt 601 and 602).
Interesting.
I never knew the Dutch AR10s had longer gas keys.
They also have 3 vent holes.
Found a pic.
http://i220.photobucket.com/albums/dd74/eyegun/AR-10/IMG_2598.jpg
http://i220.photobucket.com/albums/dd74/eyegun/AR-10/IMG_2597.jpg
Notice the forward facing exhaust ports.
I've seen pictures of early AR-15 that had longer keys than what's current now. There is some room to extend the keys somewhat.
UCWRG vent holes. Ill get a measurement in a minute.
http://i65.photobucket.com/albums/h213/gripgc8/97DE3C64-C542-4581-BB06-8E50248DD68B_zpssk2gxu0r.jpg
jerrysimons
01-04-15, 12:37
Let's discuss a little about altering the cam path in the carrier. We can start really basic and proceed to more complex later.
Sullivan was mentioned a little earlier, he put together a project with others for multiple possible improvements in the platform. One of his improvements was a double cut in the cam way to increase carrier travel before the unlocking of the bolt begins. Basically keeping the same cam cut, and superimposing an identical cut offset to slightly increase the carrier stroke before the radial cut initiates cam pin/bolt rotation. Say we build a carrier that is otherwise 100% to the TDP, except this extended cut. Use this modified carrier, with all other TDP parts for their intended roles. 20" rifles, 14.5" and 10.5" carbines as a baseline that can be expanded somewhat later.
How do you suppose the characteristics would be in general?
During the action cycling sequence, the BCG would be able to travel further with only action resistance before initiating unlocking. That increased distance takes time and allows more pressure drop in the chamber, easing unlocking, and reducing stress on the bolt. What else is going on though? The initial travel with only action resistance allows the carrier to accelerate to a higher speed before unlocking. The added time decreases chamber pressure during unlocking, thus requiring less work to operate. With less work, the carrier speed isn't slowed as much as it would if it required the original work, again additional carrier velocity. So in general, during cycling, this would be a faster carrier.
How would this effect a 20" rifle? Out of the 3 rifle samples, the 20" should see the least gain in carrier velocity. It already unlocks at a reduced pressure than the carbines, plus has increased reciprocating mass over them. This would help keep the velocity increase to a minimum.
What about the 14.5" carbine? The original BCG was intended for 20" inch guns. When used in a 14.5", unlocking occurs during a higher chamber pressure than the 20". The system is gassed to give it more energy to cycle the action, as the the unlocking requires more work to be done. How does this effect the 14.5" with the altered carrier? There would be a larger jump in carrier velocity compared to the 20", as the system is gassed harder and has less reciprocating mass.
What about the 10.5" carbine? This would be more of the same as the 14.5", just even more of a carrier velocity than the others.
So, what could be done to reduce the speeds in these instances? Dedicated gas metering would probably be a good direction to look into, if it takes less work, you don't need as much gas obviously. There's other benefits of needing less gas, fouling being one of them. Another option on the carbines is to make their action system rifle like to keep the speed in check (think A5). Really a combination of the two is probably the best solution for carbines.
Let's chew on this for a little while.
Boom! Holy Mother of all posts! This sums up about ten different threads on the LMT E-Carrier and what has been a running M4 Carbine community experiment going on several years with the mysterious, and confusingly marketed LMT e-carrier. You are the man Tom!
Allow me to illustrate the e-carrier example:
In order top to bottom:
Jp low mass
Noveske
LMT E-carrier Gen 5
Noveske
http://i8.photobucket.com/albums/a24/accidentprone86/2557D470-9DA7-4F31-A2DB-F6DA1324613E-153-00000004D9057B47_zps2c932996.jpg
http://i8.photobucket.com/albums/a24/accidentprone86/CF910BD4-625E-4292-A833-FABF2E1C905A-153-000000050408C7DF_zps782f7fcf.jpg
http://i8.photobucket.com/albums/a24/accidentprone86/0592CCDC-5D43-4457-BD1A-3EBC92A738E4-153-0000000536F3155A_zps3f1b7e0a.jpg
http://i8.photobucket.com/albums/a24/accidentprone86/A8FEF9B5-8DFB-4EBB-9E00-36838C70AA86-153-0000000672AAC637_zps8f2d3497.jpg
http://i8.photobucket.com/albums/a24/accidentprone86/50E447C7-5C07-49F6-8BD6-508A2478EA0A-153-00000006BAB7118D_zpsd76dfd38.jpg
http://i8.photobucket.com/albums/a24/accidentprone86/61770F14-A414-43C7-8A15-4A8482B2CD2A-153-00000006E838AFC3_zpsf67325a4.jpg
Track looks a little longer with a less abrupt bump to my eyes, staying locked longer and unlocking slower once it does start to twist the bolt.
https://www.m4carbine.net/showthread.php?150517-New-Drop-in-adjustable-gas-system/page2
DreadPirateMoyer
01-04-15, 12:48
Let's discuss a little about altering the cam path in the carrier. We can start really basic and proceed to more complex later.
Sullivan was mentioned a little earlier, he put together a project with others for multiple possible improvements in the platform. One of his improvements was a double cut in the cam way to increase carrier travel before the unlocking of the bolt begins. Basically keeping the same cam cut, and superimposing an identical cut offset to slightly increase the carrier stroke before the radial cut initiates cam pin/bolt rotation. Say we build a carrier that is otherwise 100% to the TDP, except this extended cut. Use this modified carrier, with all other TDP parts for their intended roles. 20" rifles, 14.5" and 10.5" carbines as a baseline that can be expanded somewhat later.
How do you suppose the characteristics would be in general?
During the action cycling sequence, the BCG would be able to travel further with only action resistance before initiating unlocking. That increased distance takes time and allows more pressure drop in the chamber, easing unlocking, and reducing stress on the bolt. What else is going on though? The initial travel with only action resistance allows the carrier to accelerate to a higher speed before unlocking. The added time decreases chamber pressure during unlocking, thus requiring less work to operate. With less work, the carrier speed isn't slowed as much as it would if it required the original work, again additional carrier velocity. So in general, during cycling, this would be a faster carrier.
How would this effect a 20" rifle? Out of the 3 rifle samples, the 20" should see the least gain in carrier velocity. It already unlocks at a reduced pressure than the carbines, plus has increased reciprocating mass over them. This would help keep the velocity increase to a minimum.
What about the 14.5" carbine? The original BCG was intended for 20" inch guns. When used in a 14.5", unlocking occurs during a higher chamber pressure than the 20". The system is gassed to give it more energy to cycle the action, as the the unlocking requires more work to be done. How does this effect the 14.5" with the altered carrier? There would be a larger jump in carrier velocity compared to the 20", as the system is gassed harder and has less reciprocating mass.
What about the 10.5" carbine? This would be more of the same as the 14.5", just even more of a carrier velocity than the others.
So, what could be done to reduce the speeds in these instances? Dedicated gas metering would probably be a good direction to look into, if it takes less work, you don't need as much gas obviously. There's other benefits of needing less gas, fouling being one of them. Another option on the carbines is to make their action system rifle like to keep the speed in check (think A5). Really a combination of the two is probably the best solution for carbines.
Let's chew on this for a little while.
This has been my experience as well. It both eases extraction and increases carrier speed. My LMT enhanced BCGs always seem to use a 1-step heavier buffer than I've seen others use on this forum with standard carriers and blue Sprinco springs: H2 buffers in 14.5 middies, and H3 buffers in 16 middies. Extraction/ejection is smoother, though (my brass ejects much more consistently) and it definitely improves performance with a suppressor.
BufordTJustice
01-04-15, 13:14
Interesting.
I never knew the Dutch AR10s had longer gas keys.
They also have 3 vent holes.
Found a pic.
http://i220.photobucket.com/albums/dd74/eyegun/AR-10/IMG_2598.jpg
http://i220.photobucket.com/albums/dd74/eyegun/AR-10/IMG_2597.jpg
Vetty interesting. This seems like it could be an inexpensive upgrade for the SBR/suppresed crowd.
New screws and a melonited, extended carrier key could be very cost effective when paired with an enhanced carrier (such as the LMT) and the Vltor A5 buffer system.
BufordTJustice
01-04-15, 13:21
Let's discuss a little about altering the cam path in the carrier. We can start really basic and proceed to more complex later.
Sullivan was mentioned a little earlier, he put together a project with others for multiple possible improvements in the platform. One of his improvements was a double cut in the cam way to increase carrier travel before the unlocking of the bolt begins. Basically keeping the same cam cut, and superimposing an identical cut offset to slightly increase the carrier stroke before the radial cut initiates cam pin/bolt rotation. Say we build a carrier that is otherwise 100% to the TDP, except this extended cut. Use this modified carrier, with all other TDP parts for their intended roles. 20" rifles, 14.5" and 10.5" carbines as a baseline that can be expanded somewhat later.
How do you suppose the characteristics would be in general?
During the action cycling sequence, the BCG would be able to travel further with only action resistance before initiating unlocking. That increased distance takes time and allows more pressure drop in the chamber, easing unlocking, and reducing stress on the bolt. What else is going on though? The initial travel with only action resistance allows the carrier to accelerate to a higher speed before unlocking. The added time decreases chamber pressure during unlocking, thus requiring less work to operate. With less work, the carrier speed isn't slowed as much as it would if it required the original work, again additional carrier velocity. So in general, during cycling, this would be a faster carrier.
How would this effect a 20" rifle? Out of the 3 rifle samples, the 20" should see the least gain in carrier velocity. It already unlocks at a reduced pressure than the carbines, plus has increased reciprocating mass over them. This would help keep the velocity increase to a minimum.
What about the 14.5" carbine? The original BCG was intended for 20" inch guns. When used in a 14.5", unlocking occurs during a higher chamber pressure than the 20". The system is gassed to give it more energy to cycle the action, as the the unlocking requires more work to be done. How does this effect the 14.5" with the altered carrier? There would be a larger jump in carrier velocity compared to the 20", as the system is gassed harder and has less reciprocating mass.
What about the 10.5" carbine? This would be more of the same as the 14.5", just even more of a carrier velocity than the others.
So, what could be done to reduce the speeds in these instances? Dedicated gas metering would probably be a good direction to look into, if it takes less work, you don't need as much gas obviously. There's other benefits of needing less gas, fouling being one of them. Another option on the carbines is to make their action system rifle like to keep the speed in check (think A5). Really a combination of the two is probably the best solution for carbines.
Let's chew on this for a little while.
Tom, holy shit this post should be a sticky. You and Clint. Just amazing.
We are not to the LMT enhanced carrier just yet, still just a very basic double cut cam pathway.
One step further on this.
"How little or far can you go with this?"
"Can this induce issues?"
Remember, this is still a over simplified discussion on this initially.
As was mentioned earlier, retaining the extractor pin becomes an issue. A retaining detent for the extractor, like the KAC E3 type is probably best. LMT used a different method that extended the front of the carrier into were the barrel extension taper is located. The LMT type limits the amount of cam path travel, as the barrel extension taper is only so deep.
Crap, I just lost 90% of my post!
Oh well, I'll start again in a little while.
One step further on this.
"How little or far can you go with this?"
"Can this induce issues?"
Remember, this is still a over simplified discussion on this initially.
As was mentioned earlier, retaining the extractor pin becomes an issue. A retaining detent for the extractor, like the KAC E3 type is probably best. LMT used a different method that extended the front of the carrier into were the barrel extension taper is located. The LMT type limits the amount of cam path travel, as the barrel extension taper is only so deep.
Ok, to continue on, this is shorter than what I had before by a lot, and I hope some of the points I'm going to jump over will be obvious now or so a little later.
If the "double cut" is relatively short, as in the extractor pin is retained in the LMT Enhanced style carrier style, there are items to consider. One of these items is that extra gas would still be needed for carbines, though the 20" rifle would be much closer to the point that additional carrier velocity reduces somewhat over the standard BCG. Ideally the cam pathway would be designed so that little or no additional carrier velocity is induced by lengthening the initial straight path with the various barrel lengths and gas systems used, etc.
To gain additional travel, alternative extractor pin retention is needed. When the LMT style enhanced carrier is used, there is "pro" to this as less special parts are needed. To go further, one of the first things that happens is that cam path interferes with the ledge that the charging handle contacts to operate the system. To extend the pathway, a different charging handle must be used that uses less carrier space to give space for the cam path.
So, optimally, is a double cut cam path the best choice? No, the angles of the cut and free space can lose positive control of the cam pin, that can lead to issues controlling bolt rotation that effect reliability.
One step further on this.
"How little or far can you go with this?"
"Can this induce issues?"
Remember, this is still a over simplified discussion on this initially.
As was mentioned earlier, retaining the extractor pin becomes an issue. A retaining detent for the extractor, like the KAC E3 type is probably best. LMT used a different method that extended the front of the carrier into were the barrel extension taper is located. The LMT type limits the amount of cam path travel, as the barrel extension taper is only so deep.
Is the need to capture the extractor pin that great? Seems to me that it shouldn't want to work it's way out on it's own, unless its just a matter of removing Murphy from the equation. Or could it be a case where the centrifugal force is enough to cause the extractor pin to move? Perhaps due to the fact that it is not centered on the axis of rotation?
Is the need to capture the extractor pin that great? Seems to me that it shouldn't want to work it's way out on it's own, unless its just a matter of removing Murphy from the equation. Or could it be a case where the centrifugal force is enough to cause the extractor pin to move? Perhaps due to the fact that it is not centered on the axis of rotation?
It's not centrifugal force for the extractor pin retention issues, it's the acceleration and deceleration that can spit the pin out enough to potentially cause problems.
Magic_Salad0892
01-04-15, 16:55
This thread needs to be stickied.
By the way. I can confirm what was said about easing extraction and needing less gas to cycle the gun with the Enhanced Carrier, and using the VLTOR A5 system. My SBRs have small ports. They are cut down Colt SOCOM barrels with standard .062'' gas ports. You can go even smaller than that. I'd be very interested to see somebody fabricate up a longer gas key. Also, what's with the indent on the Dutch AR10 carrier? Do their AR15 carriers have the same thing? How does hard chroming factor in?
Vetty interesting. This seems like it could be an inexpensive upgrade for the SBR/suppresed crowd.
New screws and a melonited, extended carrier key could be very cost effective when paired with an enhanced carrier (such as the LMT) and the Vltor A5 buffer system.
There have been versions of this made that even stretch out to the point that the key never looses it's interface with the tube. Issues have been fouling that collects from blow by dragging on the system that need to be overcome.
Magic_Salad0892
01-04-15, 16:57
There have been versions of this made that even stretch out to the point that the key never looses it's interface with the tube. Issues have been fouling that collects from blow by dragging on the system that need to be overcome.
Decreasing gas port size, and increasing the size of the exhaust ports would fix that, wouldn't it?
Decreasing gas port size, and increasing the size of the exhaust ports would fix that, wouldn't it?
Help, yes. Fix it, no. The gasses pressurizing the BCG and making a 180 turn for that short moment leave the condensed deposits.
The A5 with the LMT Enhanced carrier is a great improvement over the std carrier and carbine type action system with reduced gas, but it's not as good as what can be done on the system.
BufordTJustice
01-04-15, 18:54
The A5 with the LMT Enhanced carrier is a great improvement over the std carrier and carbine type action system with reduced gas, but it's not as good as what can be done on the system.
Tom, does the KAC SR15 carrier have a std cam pin track or an altered one?
Tom, does the KAC SR15 carrier have a std cam pin track or an altered one?
The standard one.
jerrysimons
01-04-15, 19:06
The A5 with the LMT Enhanced carrier is a great improvement over the std carrier and carbine type action system with reduced gas, but it's not as good as what can be done on the system.
Is this because the e-carrier actually gains some rearward velocity due to 1) accelerating straight-back slightly longer before the cam pin is engaged laterally by the jaunt in the cam pin track that twists and unlocks the bolt, and 2) Once the bolt starts to unlock and extract the case there is less pressure inside the empty case/chamber (due to the delay) that pushes the case walls against the chamber walls and thus causes less friction which allows for easier extraction? If this is the case the e-carrier can get away with venting more gas but didn't you say that "extra gas would still be needed for carbines"? Maybe I misunderstood this phrase. Please correct me if I am wrong.
BufordTJustice
01-04-15, 19:16
Is this because the e-carrier actually gains some rearward velocity due to 1) accelerating straight-back slightly longer before the cam pin is engaged laterally by the jaunt in the cam pin track that twists and unlocks the bolt, and 2) Once the bolt starts to unlock and extract the case there is less pressure inside the empty case/chamber (due to the delay) that pushes the case walls against the chamber walls and thus causes less friction which allows for easier extraction? If this is the case the e-carrier can get away with venting more gas but didn't you say that "extra gas would still be needed for carbines"? Maybe I misunderstood this phrase. Please correct me if I am wrong.
Basically yes. In simple terms, less of the energy budget of the recoil stroke is used up in the extraction of the case and is instead delivered to the action spring/receiver extension.
Crap, I just lost 90% of my post!
Oh well, I'll start again in a little while.
That is the damn worst! I keep notepad up if I end up with a huge post that I copy / paste my text into to help keep me from shooting my computer in the event it does that to me.
Also sub'd to this thread, glad you guys are dishing out this brain food!
Just opened a brand new BCM bcg, and brand new LMT enhanced carrier......... BCM bolt won't fit in the carrier
DreadPirateMoyer
01-05-15, 13:34
Just opened a brand new BCM bcg, and brand new LMT enhanced carrier......... BCM bolt won't fit in the carrier
This has happened to me before as well, except with a Colt bolt. It should work, though. You just need to give it a lot more umph (and lube) than you're used to. A hundred rounds later and it'll work just fine.
BufordTJustice
01-05-15, 13:37
Just opened a brand new BCM bcg, and brand new LMT enhanced carrier......... BCM bolt won't fit in the carrier
It's supposed to be very firm in fitment until it's been lubed well and shot. My my LMT bolt was difficult to seat inside my gen 4 enhanced carrier when i got them.
Just to be safe, check the gas rings for burrs.
Worked it manually for a while, Got it in, we wondered if it would cycle, it did at indoor range with frangible. (with vltor a5 h3 on a 11.5 bcm)
Now going to try it with my 556 saker
BufordTJustice
01-05-15, 16:15
Worked it manually for a while, Got it in, we wondered if it would cycle, it did at indoor range with frangible. (with vltor a5 h3 on a 11.5 bcm)
Now going to try it with my 556 saker
You're good brother. Just keep it nice and wet with lube through the gas exhaust vent ports.
If you really want to dive deep, and look at what was going on with the 1959-1960 Colt AR10A, there are a lot of features on the Colt AR10A bolt carrier group that will show how ahead of the time they were. Colt spent $100,000 tooling up for full production of the AR10A, but put the brakes on that and shifted to AR15 production before they could start cranking out AR10's.
* Shorter receiver set, which we haven't seen again until the Colt 901
* Reduced length carrier key base, but with long carrier key gas tube sleeve
* Single carrier key fastener
* Roller cam pin
* Roller cam pin relief cut in gas key sleeve
* Dutch paratrooper style charge handle
* And what's that tiny little hole on the left side of the carrier, forward of the old school FPRPin retainer?
* The long groove is for a belt-fed option.
http://i13.photobucket.com/albums/a264/artweaker/Thread%20content/Books/DSCN3494.jpg~original
http://i225.photobucket.com/albums/dd297/armeiro1/MP28II/ar-10a.jpg
sinlessorrow
01-08-15, 00:10
If you really want to dive deep, and look at what was going on with the 1959-1960 Colt AR10A, there are a lot of features on the Colt AR10A bolt carrier group that will show how ahead of the time they were. Colt spent $100,000 tooling up for full production of the AR10A, but put the brakes on that and shifted to AR15 production before they could start cranking out AR10's.
* Shorter receiver set, which we haven't seen again until the Colt 901
* Reduced length carrier key base, but with long carrier key gas tube sleeve
* Single carrier key fastener
* Roller cam pin
* Roller cam pin relief cut in gas key sleeve
* Dutch paratrooper style charge handle
* And what's that tiny little hole on the left side of the carrier, forward of the old school FPRPin retainer?
* The long groove is for a belt-fed option.
http://i13.photobucket.com/albums/a264/artweaker/Thread%20content/Books/DSCN3494.jpg~original
http://i225.photobucket.com/albums/dd297/armeiro1/MP28II/ar-10a.jpg
No matter what people say Colt has been an innovator on the AR platform for a long time. I am glad they focused on the AR-15, though the 901 is a superb evolution in the AR-10 rifle.
A lot of those improvements to the AR10 came from the Dutch, who had purchased the licensed manufacturing rights for the Fairchild-ArmaLite AR10 over a 5-year period of manufacture, from 1957-1962. They looked at the charge handle, the carrier key, the ejector, and vent holes on the carrier. The first ArmaLite prototypes that Stoner built had vent holes cut at 90* angles from the bore axis.
The Dutch used angled vent holes. One would have to model laminar flow in the expansion chamber formed by the bolt tail, inside of the carrier, to see how that offers any benefits, but the Dutch guns are one of the highest quality battle rifle samples in the world. They compare easily to the Sig-510, and the attention to detail is exceptional.
I'm guessing there is more of a staged venting of gases out of the carrier through the angled vent holes, rather than a quick drop in pressure.
What's interesting about the Colt AR10A is that ArmaLite seems to have taken some of the Dutch tweaks, reduced the length of the receivers and bolt carrier group for a more compact AR10, and used AR15 FCG parts. You can clearly see the AR15 hammer, as opposed to the original coil spring-driven hammer from the AR10's. Looks like the transitional-era Dutch/ArmaLite/Colt AR10A was 55 years ahead of the new trends in shorter AR10's.
If you really want to dive deep, and look at what was going on with the 1959-1960 Colt AR10A, there are a lot of features on the Colt AR10A bolt carrier group that will show how ahead of the time they were. Colt spent $100,000 tooling up for full production of the AR10A, but put the brakes on that and shifted to AR15 production before they could start cranking out AR10's.
* Shorter receiver set, which we haven't seen again until the Colt 901
* Reduced length carrier key base, but with long carrier key gas tube sleeve
* Single carrier key fastener
* Roller cam pin
* Roller cam pin relief cut in gas key sleeve
* Dutch paratrooper style charge handle
* And what's that tiny little hole on the left side of the carrier, forward of the old school FPRPin retainer?
Pretty sure thats a roll pin bore to retain the carrier key screw instead of staking
* The long groove is for a belt-fed option.
http://i13.photobucket.com/albums/a264/artweaker/Thread%20content/Books/DSCN3494.jpg~original
http://i225.photobucket.com/albums/dd297/armeiro1/MP28II/ar-10a.jpg
Added in that the small hole should be a roll pin bore.
Good call, and I think that makes sense. They were doing that type of retention method back then, which is most evident on the extension tube roll pin visible on the lower receiver. That transferred over to several of the production guns, including the Colt 601 AR15, 602, 603/XM16E1, Colt 605, and any model that had the partial fence lower receiver before the finalized version of the 603 (M16A1) and 604 (USAF M16) went into production.
If you look at the new Colt 901, they went back to the AR10A bolt carrier set-up, and their carrier key is much shorter, with fasteners located more closely together. Also notice the 3 large vent holes on the LE901, and their location:
http://www.gunsandammo.com/wp-content/blogs.dir/2/files/go-big-or-go-home-the-colt-le901-16s-review/colt-le901-16s_005.jpg
Robb Jensen
01-09-15, 05:27
I see the difference in exhaust ports on carriers vary but I'm not sure it matters much. Where I see huge differences are in the size of the port on the top of the carrier where it's getting the gas from the carrier key. It ranges all over the place but never seems to be smaller than the hole in the bottom of the carrier key.
Sometimes I believe when the hole in the top of the carrier is way big and the key is smaller I think it's more likely to leak especially if not torqued tight enough or if the carrier key isn't completely flat and sealed well. Some carrier keys are sometimes a bit wide and don't fit down well into the machined channel on the carrier key seat and also cause leaks.
sinlessorrow
01-10-15, 22:37
Got my Colt BCG in today for a new build and there is a huge difference. MP4 BCG has .101 gas vents and they are farther apart and angled different, Colt has .110 gas ports closer together and angled different. I also noticed a few other differences.
http://i203.photobucket.com/albums/aa273/SinlesSorrow/Mobile%20Uploads/0110152147_zps876fe513.jpg
The other differences I saw where in the bottom rails on the Colt which are ground flat instead of sharp, and the black extractor insert is different. You can tell it is a more stout material than even other black extractor inserts.
http://i203.photobucket.com/albums/aa273/SinlesSorrow/Mobile%20Uploads/0110152154_zpsb83de5b3.jpg
I don't know if this is a new thing or not, but it is a couple of differences from my MP4 and BCM BCG's.
BufordTJustice
01-11-15, 09:11
Got my Colt BCG in today for a new build and there is a huge difference. MP4 BCG has .101 gas vents and they are farther apart and angled different, Colt has .110 gas ports closer together and angled different. I also noticed a few other differences.
http://i203.photobucket.com/albums/aa273/SinlesSorrow/Mobile%20Uploads/0110152147_zps876fe513.jpg
The other differences I saw where in the bottom rails on the Colt which are ground flat instead of sharp, and the black extractor insert is different. You can tell it is a more stout material than even other black extractor inserts.
http://i203.photobucket.com/albums/aa273/SinlesSorrow/Mobile%20Uploads/0110152154_zpsb83de5b3.jpg
I don't know if this is a new thing or not, but it is a couple of differences from my MP4 and BCM BCG's.
Thanks for the info. Can you grab a close up of the differences in the angle of the ports themselves?
sinlessorrow
01-11-15, 12:05
Yeah I'll get one tonight.
Another difference in vents is location as well. The Colt as well as my BCM carriers have the vents slightly forward so that when the bolt is fully extended the tail of the bolt still covers half of the vent as seen here.
http://i203.photobucket.com/albums/aa273/SinlesSorrow/Mobile%20Uploads/0111151147a_zpsf918f8f8.jpg
On my MP4 carriers, the vents are smaller but also more rearward so that the tail goes forward enough that the exhaust vent is fully unobstructed.
Now what difference this makes I have no idea. I have a hunch it means the carrier of my MP4 depressurizes faster and sooner than my Colt and BCM carriers as it will vent the gas sooner.
BufordTJustice
01-11-15, 12:59
Yeah I'll get one tonight.
Another difference in vents is location as well. The Colt as well as my BCM carriers have the vents slightly forward so that when the bolt is fully extended the tail of the bolt still covers half of the vent as seen here.
http://i203.photobucket.com/albums/aa273/SinlesSorrow/Mobile%20Uploads/0111151147a_zpsf918f8f8.jpg
On my MP4 carriers, the vents are smaller but also more rearward so that the tail goes forward enough that the exhaust vent is fully unobstructed.
Now what difference this makes I have no idea. I have a hunch it means the carrier of my MP4 depressurizes faster and sooner than my Colt and BCM carriers as it will vent the gas sooner.
THAT is very interesting. You're correct that hole location would have an effect on the timing and rate of depressurization. Not sure how much of an effect this would have on actual operation. But this could be another small rabbit hole.
sinlessorrow
01-11-15, 13:26
THAT is very interesting. You're correct that hole location would have an effect on the timing and rate of depressurization. Not sure how much of an effect this would have on actual operation. But this could be another small rabbit hole.
I have to give it to BCM though. comparing their BCG to COLT I can see no difference, something I cannot say of any other BCG I have used.
BufordTJustice
01-11-15, 14:35
I have to give it to BCM though. comparing their BCG to COLT I can see no difference, something I cannot say of any other BCG I have used.
Yeah, that doesn't happen by accident.
I would personally prefer if BCM didn't use CMT cam pins though. The CMT cam pins tend to wear uppers more and break more often in the firing pin hole area than others. Just a visual check shows that they relieve the firing pin hole coarsely and they have sharper edges at the top of the pin. Some complain about how LMT cam pins wear a groove, but they don't wear other components like the CMT have. I have swapped out CMT cam pins out of many "premium" builds and throw in a few LMT pins for many, including Noveske.
Just to expand a bit about the cam path altering touched on earlier.
What happens if the pre travel in the cam event is reduced via BCG modifications, leaving the other items the same as TDP? With all safety concerns dealt with, obviously.
How does this alter the operation of the system as a whole work when concerning the 3 examples of 20" rifle gas, 14.5" carbine gas, and 10.5" carbine gas?
I won't write too much, as I tend to lose posts here.
If done incrementally some trends seem to show.
The 20" does require more gas to operate compared to a std TDP BCG.
The 14.5" requires more, while the 10.5" requires more than the 14.5".
Brass stretch and visible extractor pull marks become much more obvious as the pre travel shortens.
The same can be shown with both rifle and carbine actions for the listed barrel lengths.
So, what to make of all of this about the cam path?
Is it ideal in the TDP? No.
Can options be made to change some of the issues? More than likely so.
So, from here, what can be done to look into the options we have available, like the LMT Enhanced Carrier and others?
BufordTJustice
01-11-15, 17:15
I would personally prefer if BCM didn't use CMT cam pins though. The CMT cam pins tend to wear uppers more and break more often in the firing pin hole area than others. Just a visual check shows that they relieve the firing pin hole coarsely and they have sharper edges at the top of the pin. Some complain about how LMT cam pins wear a groove, but they don't wear other components like the CMT have. I have swapped out CMT cam pins out of many "premium" builds and throw in a few LMT pins for many, including Noveske.
I've been using LMT cam pins until i switched to NiB cam pins. Those don't wear either the upper or the bolt as much. But, now, I'm using the V7 cam pin and I don't think I'll be using anything else from here on out.
Not to sidebar this awesome thread but H&K cam pins... You won't go back.
Not to sidebar this awesome thread but H&K cam pins... You won't go back.
Where did you find these?
BufordTJustice
01-12-15, 16:38
Not to sidebar this awesome thread but H&K cam pins... You won't go back.
Pix please. :D
sinlessorrow
01-12-15, 16:59
Pix please. :D
From all looks it is just a basic cam pin that costs $20.
http://www.hkparts.net/shop/pc/catalog/general/hknghk2539.jpg
It could be great, it could be mediocre, I don't know? I don't doubt HK's quality standards, but the top of the pin seems to lack a more rounded radius on the corners than what I prefer, but it's just a picture and not in my hands.
No gas ports, obviously, but a nice view of a real HK cam pin.
http://hkguns.zenfolio.com/img/s6/v145/p851201376-5.jpg
BufordTJustice
01-12-15, 21:19
No gas ports, obviously, but a nice view of a real HK cam pin.
http://hkguns.zenfolio.com/img/s6/v145/p851201376-5.jpg
I'm sure it's of typical HK quality.....but i prefer the rounded corners of the V7.
sinlessorrow
03-23-15, 01:59
Just an update.
According to colt it varies between .110 and .120. The rear carrier/top carrier in this pic has .120" vent ports, and the front carrier/bottom carrier has .110" vent ports.
http://i.imgur.com/wYayweI.jpg
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