View Full Version : Measuring the barrel gas port size
While I am generally opposed to the amount of attention paid to this level of minutiae, it strikes me that if we're going to discuss it so often we should determine how people are making their measurements and assumptions.
As I understand it the proper method is to use a set like these (http://www.nolansupply.com/bysubcategory.asp?category=Precision+Tools&supercategory=Gage+Pins&subcategory=Meyer+Gage+Pin+Sets&type=False&specs=True).
Further, I understand the "plus" and "minus" to mean that they are just slightly over, or under, sized relative to the stated diameter of the pin. 0.0002" is the over/under I've seen reported. So a 0.060" minus pin would just barely fit in a true 0.060" hole while a plus would not.
Additionally, I think that if you are the one reporting a hole size that you measured you should be stating how you took the measurement, and if you are repeating a size you saw reported somewhere else you should be linking to that other place or at the very least stating your source.
Repliers should give the manufacturer and the port size so a list can be created for all future, "What is the port size of a ...." questions.
If it's relevant to this thread, open a discussion of what type of affects different port sizes have on the functioning and cycling of a/their weapon, ie.. suppressed/unsuppressed, over gassed/ under gassed, FTE, etc.
This could become the official gas PORT thread!
Gas PORT not gas BLOCK. Gas blocks tend to have larger holes to accommodate all barrel port sizes.
Gas PORT not gas BLOCK. Gas blocks tend to have larger holes to accommodate all barrel port sizes.
:eek: Typed the wrong word. Editing. :suicide:
While I am generally opposed to the amount of attention paid to this level of minutiae, it strikes me that if we're going to discuss it so often we should determine how people are making their measurements and assumptions.
As I understand it the proper method is to use a set like these (http://www.nolansupply.com/bysubcategory.asp?category=Precision+Tools&supercategory=Gage+Pins&subcategory=Meyer+Gage+Pin+Sets&type=False&specs=True).
Further, I understand the "plus" and "minus" to mean that they are just slightly over, or under, sized relative to the stated diameter of the pin. 0.0002" is the over/under I've seen reported. So a 0.060" minus pin would just barely fit in a true 0.060" hole while a plus would not.
Additionally, I think that if you are the one reporting a hole size that you measured you should be stating how you took the measurement, and if you are repeating a size you saw reported somewhere else you should be linking to that other place or at the very least stating your source.
This level of minutia is probably more relevant to how the carbine functions than a lot of other things that folks obsess over on this board.
While I am generally opposed to the amount of attention paid to this level of minutiae, it strikes me that if we're going to discuss it so often we should determine how people are making their measurements and assumptions.
As I understand it the proper method is to use a set like these (http://www.nolansupply.com/bysubcategory.asp?category=Precision+Tools&supercategory=Gage+Pins&subcategory=Meyer+Gage+Pin+Sets&type=False&specs=True).
Further, I understand the "plus" and "minus" to mean that they are just slightly over, or under, sized relative to the stated diameter of the pin. 0.0002" is the over/under I've seen reported. So a 0.060" minus pin would just barely fit in a true 0.060" hole while a plus would not.
Additionally, I think that if you are the one reporting a hole size that you measured you should be stating how you took the measurement, and if you are repeating a size you saw reported somewhere else you should be linking to that other place or at the very least stating your source.
Feeler gauges are the way. If you cannot pony up the cash, drill bits are not a bad way to get a GENERAL idea.
Still another way (along with calipers) is to take a punch and grind it down to the hole size and then measure it.
C4
This level of minutia is probably more relevant to how the carbine functions than a lot of other things that folks obsess over on this board.
Correct.
C4
Much better prices on these pin guages. (http://www.auto-met.com/gage/pin_gage.htm)
Just get 8-10 around the diameter you are trying to measure (e.g., around .062") and go from there. If your largest guage fits in the port, then it's prety obvious you are overgassed, and this should lead you to playing with heavier buffers/spings/carriers, barrel replacement, or a weld up and re-drill job.
Magic_Salad0892
08-31-11, 08:48
I take it a digital calliper wouldn't work...
This level of minutia is probably more relevant to how the carbine functions than a lot of other things that folks obsess over on this board.
Functions? Maybe. But frankly this obsession with brakes/gas ports/springs/buffers is inconsequential if one simply buys a quality firearm and quits ****ing with it to begin with. A grip/stock/handguard that fits me and my shooting style is going to have way more to do with getting rounds on target that garage-re-engineering the system that had nothing wrong with it to begin with to try and tune where my brass lands.
I take it a digital calliper wouldn't work...
You can get an "idea," but generally no.
C4
Much better prices on these pin guages. (http://www.auto-met.com/gage/pin_gage.htm)
Just get 8-10 around the diameter you are trying to measure (e.g., around .062") and go from there. If your largest guage fits in the port, then it's prety obvious you are overgassed, and this should lead you to playing with heavier buffers/spings/carriers, barrel replacement, or a weld up and re-drill job.
Interesting, so for the range we're talking about they are $2.00 ea and you order them by the piece for the sizes you're looking for? What increments would you buy? 0.0620 through what size at the high end, in what increments? It appears they sell them in 0.0005 increments. I'm assuming you're not suggesting:
0.0620
0.0625
0.0630
0.0635
etc.
but perhaps
0.0620
0.0630
0.0640
0.0650
etc.
?
Feeler gauges are the way.
Feeler gauges? I thought feeler gauges were flat metal used for measuring gaps?
http://www.diseno-art.com/images_2/feeler_gauge.jpg
I would also like to hear from people that have measured, or think they have measured, some ports and what kind of size range you've encountered. I've seen posts mentioning down to 0.060 and up to 0.090, which would be a total of 31 pins at .001 increments. I'd hate to buy $60 worth of pins only to find out I need one two sizes smaller, or two sizes larger. :suicide2:
I have no way of verifying/vouching but this post has some interesting information.
http://arizonashooting.com/v3/viewtopic.php?f=8&t=12922
Interesting, so for the range we're talking about they are $2.00 ea and you order them by the piece for the sizes you're looking for? What increments would you buy? 0.0620 through what size at the high end, in what increments? It appears they sell them in 0.0005 increments. I'm assuming you're not suggesting:
0.0620
0.0625
0.0630
0.0635
etc.
but perhaps
0.0620
0.0630
0.0640
0.0650
etc.
?
The latter. And I'd probably buy them in .002" steps, e.g., .062, .064, .066, .068, .070, .072. I seriously doubt .001 makes that much of a difference, but it may be worth $25 to find out if your port is .010" or more over spec.
And regarding my earlier point, I'm in agreement with you with respect to brakes and other add-ons. But if you are having trouble with functioning or you're engaging in a build, knowing your gas port diameter is a good number to have so you can go in the right direction.
I would suggest you probably wouldn't be well served if you are thinking of purchasing a Colt or BCM, but if you are looking at a build and buying things pieces at a time, it would be useful to know what you're dealing with WRT to the port, then you can make infomed decisions about springs and buffers. And for an alreday assembled carbine, it also can help isolate functioning issues and lead to to the right corrective actions.
Alternatively, just assemble from whatever parts you have on hand or shoot what you bought with whatever ammo you bought and then maybe subject yourself to having to swap out springs, buffers and mags until you find a combo that works. That can work as well, it's just time consuming, funds consuming and PITA challenging.
While I am generally opposed to the amount of attention paid to this level of minutiae
Just curious as to why the opposition? This forum has plenty of threads discussing the finer points of H vs. H2 buffers, ejector springs and O-rings, and so on.
Is the resistance because the gas port dimension is thought to be proprietary knowledge? I'll admit that properly* measuring a gas port is harder to do than reading the H stamp on a buffer, but it's not industrial espionage or "reverse engineering" (as some industry professionals on this board have whined). You own the rifle. It's your bitch. Measuring it ain't no crime. If the manufacturer didn't want you to measure it, they made a big mistake in the first place by selling it.
* Use pin gages, not calipers. Though a numbered drill index can give you a rough idea.
The latter. And I'd probably buy them in .002" steps, e.g., .062, .064, .066, .068, .070, .072. I seriously doubt .001 makes that much of a difference, but it may be worth $25 to find out if your port is .010" or more over spec.
And regarding my earlier point, I'm in agreement with you with respect to brakes and other add-ons. But if you are having trouble with functioning or you're engaging in a build, knowing your gas port diameter is a good number to have so you can go in the right direction.
I would suggest you probably wouldn't be well served if you are thinking of purchasing a Colt or BCM, but if you are looking at a build and buying things pieces at a time, it would be useful to know what you're dealing with WRT to the port, then you can make infomed decisions about springs and buffers. And for an alreday assembled carbine, it also can help isolate functioning issues and lead to to the right corrective actions.
Alternatively, just assemble from whatever parts you have on hand or shoot what you bought with whatever ammo you bought and then maybe subject yourself to having to swap out springs, buffers and mags until you find a combo that works. That can work as well, it's just time consuming, funds consuming and PITA challenging.
We're not far off from being on the same page then. I just sometimes think that with some of these discussions we're putting the cart before the horse and a lot of people get confused and start to think that it really matters. As we also found in one thread, most people wouldn't know what the number means even if they have it, and even if it is accurate.
What I'm hoping to do with this thread is get some accountability in these discussions since apparently we are going to have them whether I like it or not. :shout: That is why I propose that people tossing numbers around either claim those numbers as their own and tell us how they measured them, or link to the source of the information they got if they are just regurgitating. and also why I'm willing to spend a few dollars to get the gauges to measure some ports myself because they I'll KNOW I did it right and not rely on a guy jamming Home Depot drill bits into the hole in his barrel and calling it accurate to 0.001 (or 0.002 as the case may be).
Just curious as to why the opposition? This forum has plenty of threads discussing the finer points of H vs. H2 buffers, ejector springs and O-rings, and so on.
Because it's generally minutia that doesn't matter, and it misleads newer shooters into thinking that it does matter. As mentioned there was already one thread here I can recall where someone was trying to find out his gas port size with absolutely NO idea what to do with the information if he had it.
:jester:
Which doesn't let the brake/buffer/spring/o-ring people off the hook either in terms of useless minutiae.
Which doesn't let the brake/buffer/spring/o-ring people off the hook either in terms of useless minutiae.
+1 to that! :meeting:
This level of minutia is probably more relevant to how the carbine functions than a lot of other things that folks obsess over on this board.
Correct, but most people would be better served buying a quality barrel and using the buffer and spring the barrel manufacturer recommends (or just buy a complete rifle).
Sure you can "tune" a carbine by how the recoil "feels" or where the brass ejects to but cyclic rate testing is far more precise, and not an option to most.
Feeler gauges? I thought feeler gauges were flat metal used for measuring gaps?
http://www.diseno-art.com/images_2/feeler_gauge.jpg
There are flat ones and round ones. They also go by several other names.
C4
I would also like to hear from people that have measured, or think they have measured, some ports and what kind of size range you've encountered. I've seen posts mentioning down to 0.060 and up to 0.090, which would be a total of 31 pins at .001 increments. I'd hate to buy $60 worth of pins only to find out I need one two sizes smaller, or two sizes larger. :suicide2:
The mil-spec guns are going to be right around the same measurments (on the carbine side). So I would get something from say .062-.064.
For the non-milspec guns (BM, RRA, DPMA, S&W, Oly, etc), I would get ones starting at about .069-.078.
C4
Chris17404
08-31-11, 13:51
On a related note, I think it would be very interesting to have a chart of the gas port sizes for all of the various top-tier AR manufacturers, like BCM, DD, Colt, Noveske, etc. Ideally, the chart would include gas port sizes for all of their gas system/barrel length combos.
It would be even better if we could get each of the manufacturers to describe WHY they chose that gas port size.
Chris
The ones Grant is talking about are called wire feeler gauges usually used to measure spark plug/ignition gaps. I haven't found a set that goes in small increments or that big of a gap.
EzGoingKev
08-31-11, 18:12
...we should determine how people are making their measurements and assumptions.
I have Daniel Defense lightweight barrel with a mid-length gas system and the port size is .072".
I determined the size by using pin gauges -
https://lh5.googleusercontent.com/-P4aN0mhkJ2Y/Tl624SeBpgI/AAAAAAAAD2Q/f44cjdhyERg/s800/IMAG0177.jpg
And then confirmed the pin gauge sizing with a micrometer -
https://lh4.googleusercontent.com/-IWYtFtjIYGs/Tl62HmcDS-I/AAAAAAAAD2E/8kRaOIGALho/s800/IMAG0180.jpg
The funny thing is the package it came in states it has a carbine length gas system but it is a mid-length.
While I am generally opposed to the amount of attention paid to this level of minutiae....
Rob_S,
This is not an attack, I am not "butthurt", nor do I have a bone to pic but when reading that statement and thinking about some of the things you have been concerned with I rolled my eyes.
The mil-spec guns are going to be right around the same measurments (on the carbine side). So I would get something from say .062-.064.
For the non-milspec guns (BM, RRA, DPMA, S&W, Oly, etc), I would get ones starting at about .069-.078.
C4
If you're not in a hury, I'd remove the gas block/FSB and then use whatever you have on hand to "get close". Use a caliper, use a drill bit, whatever. From that dimension, then I'd measure and order gauges covering .002/4" under-to-0.002/4" over in .002" increments. That way you don't have a bunch of needless gauges on hand.
I'm no gunsmith so I'm not too keen on ordering a bunch of gauges just to have them on hand to cover the gamut of whatever might come my way. Alternatively (for carbines, not sure what I'd order for mid-lengths, so the initial measuring would be necessary) I'd just order .060" - .074" or thereabouts in .002" steps and that should cover what I'd be interested in learning. Anything gauging above my max pin gauge would let me know I'm way overgassed and I ought to be looking at heavier buffers and stiffer springs.
Again, I'm no gunsmith, but I don't know how important it is to know your exact gas port dimension, but it would seem to be important to know if you are in the range of correct port size vs over-sized. Not really sure where that demarcation would fall, but I'm pretty certain that if I knew I were .010" over spec, then I'm in the overgassed range and need to take some mitigating action. Maybe .005" over lands you in this range, maybe .007" is the number, that's another discussion. If enough people reported their:
port dimension
gas system length
buffer weight
carrier type (FA vs SA)
buffer spring type (stock vs CS extra power)
type of ammo they shoot
and how well the gun runs with all that considered together
then we'd develop a database that in time would lead us in the right direction. Maybe that's too much to ask.
This would probably be more useful for the non-Colt/BCM owners out there because you can probably assume that the Colts/BCMs are running in-spec, or very close to in-spec, ports. It would also validate whether or not the BM/RRA/Armalite/et al owners do indeed have over-gassed weapons or if these barrels too are in-spec.
If you want to measure small holes, I recommend a number drill bit set, like #60-#1 (http://www.pts-tools.com/cgi/CGPTSRIM?PMITEM=AP50S71H&PARTPG=CGPTLMXI&PAMENU=1_1&PMLFNO=01_07_006_012).
It is not exactly like a pin gauge set with sizes every thou, but works because holes are made by drill bits, in the sizes of standard drill bits, which follow the standard wire sizes.
Use the shank end to check fit and double check the diameter with a caliper or better yet, micrometer.
But honestly, if you can measure it within a thou, you are good enough for this application.
Also, I don't think this thread will make many Mfgs happy, as they consider gas port sizes proprietary and probably spent a great deal of time and ammo arriving at this number.
For example, how big is the gas port on a 14.5" BCM middy?
You won't find the answer here.
Caliber plays a big role as well, and should be noted if not 5.56.
The pressure curves of 6.8x43 or 7.62x35 are much faster than 5.56x45 and requre larger ports or closer gas port locations.
Based off of what I know some of those numbers are seriously jacked.
I have no way of verifying/vouching but this post has some interesting information.
http://arizonashooting.com/v3/viewtopic.php?f=8&t=12922
EzGoingKev
09-01-11, 05:27
http://arizonashooting.com/v3/viewtopic.php?f=8&t=12922
Barrel Length (in) Barrel Diameter (in) Distance from Muzzle (in) Min Port Size (in) Max Port Size (in)
11.5 .625 3.850 .081 .089
11.5 .750 3.850 .086 .094
14.5 .625 8.375 .063 .078
14.5 .750 8.375 .070 .086
16 .625 8.375 .063 .078
16 .750 8.375 .070 .086
20 .625 6.875 .086 .093
20 .750 6.875 .093 .096
24 .825 N/A .089 .089
This guy lists different gas port sizes for the same length barrel but with different barrel diameters.
Does that make sense?
If you want to measure small holes, I recommend a number drill bit set, like #60-#1 (http://www.pts-tools.com/cgi/CGPTSRIM?PMITEM=AP50S71H&PARTPG=CGPTLMXI&PAMENU=1_1&PMLFNO=01_07_006_012).
Number drills will get you in the ballpark, if that's all that's available.
# dia., inches
38 .1015
39 .0995
40 .0980
41 .0960
42 .0935
43 .0890
44 .0860
45 .0820
46 .0810
47 .0785
48 .0760
49 .0730
50 .0700
51 .0670
52 .0635
53 .0595
54 .0550
55 .0520
This guy lists different gas port sizes for the same length barrel but with different barrel diameters.
Does that make sense?
This is the list from the old Maryland AR-15 site, correct?
A couple of things stand out:
He's got the wrong gas port distance from the muzzle between the 14.5 and 16 inch barrels. He lists the same number, but that can't be since we all know the 16" has additional dwell.
He does not account for mid-length setups, but this list is so old, it probably pre-dates mid-lengths coming on the market.
I have no idea why gas port diameters would change between pencil barrels and standard ".750 under the gas block" barrels. I've always wanted to see an explanation on this, but when I asked years ago, I never got one.
This guy lists different gas port sizes for the same length barrel but with different barrel diameters.
Does that make sense?
Kind of, but it's not something I had considered. Thicker barrels have deeper gas port holes, so maybe he meant to even out the restriction by using a tighter hole on short holes and a wider hole on thick barrels.
The difference in hole length from a .750 to .625 barrel is only .0625". If you think about it, blowing through a long skinny straw is harder than blowing through a short skinny straw.
Would have to actually model this or set up some tests to be sure, though!
If you're not in a hury, I'd remove the gas block/FSB and then use whatever you have on hand to "get close". Use a caliper, use a drill bit, whatever. From that dimension, then I'd measure and order gauges covering .002/4" under-to-0.002/4" over in .002" increments. That way you don't have a bunch of needless gauges on hand.
I'm no gunsmith so I'm not too keen on ordering a bunch of gauges just to have them on hand to cover the gamut of whatever might come my way. Alternatively (for carbines, not sure what I'd order for mid-lengths, so the initial measuring would be necessary) I'd just order .060" - .074" or thereabouts in .002" steps and that should cover what I'd be interested in learning. Anything gauging above my max pin gauge would let me know I'm way overgassed and I ought to be looking at heavier buffers and stiffer springs.
Again, I'm no gunsmith, but I don't know how important it is to know your exact gas port dimension, but it would seem to be important to know if you are in the range of correct port size vs over-sized. Not really sure where that demarcation would fall, but I'm pretty certain that if I knew I were .010" over spec, then I'm in the overgassed range and need to take some mitigating action. Maybe .005" over lands you in this range, maybe .007" is the number, that's another discussion. If enough people reported their:
port dimension
gas system length
buffer weight
carrier type (FA vs SA)
buffer spring type (stock vs CS extra power)
type of ammo they shoot
and how well the gun runs with all that considered together
then we'd develop a database that in time would lead us in the right direction. Maybe that's too much to ask.
This would probably be more useful for the non-Colt/BCM owners out there because you can probably assume that the Colts/BCMs are running in-spec, or very close to in-spec, ports. It would also validate whether or not the BM/RRA/Armalite/et al owners do indeed have over-gassed weapons or if these barrels too are in-spec.
Most people do not need to know exact details in regards to their GP size. Just a round about idea.
Having measured enough barrels to know, people that own a RRA/BM/DPMS/Oly/S&W/Etc should accept the fact that their gun is over gassed and try to address it.
C4
militarymoron
09-01-11, 14:24
people that own a RRA/BM/DPMS/Oly/S&W/Etc should accept the fact that their gun is over gassed and try to address it.
C4
just shoot nothing but underpowered wolf through them :)
just shoot nothing but underpowered wolf through them :)
You know, I think that is reason why the cheaper guns are overgassed. Less problems with cheap ammo. Most sales are from the cheaper guns, so why not make them reliably shoot the cheaper ammo.
You know, I think that is reason why the cheaper guns are overgassed. Less problems with cheap ammo. Most sales are from the cheaper guns, so why not make them reliably shoot the cheaper ammo.
This has been "known" in the industry forever. Meaning that RRA/BM/Oly/DPMS/etc assume that their customers are going to shoot the worst ammo they can find (read under pressured). So to reduce the number of CS calls they get about bolts failing to lock back, they over gas their guns.
Now if we look at a company like LMT, they tell people that if they are shooting crap ammo, their guns aren't going to lock back (so deal with it). LMT makes guns for people that get into gun fights. ;)
I like LMT's CS advice the best.
C4
wolf_walker
09-01-11, 22:08
You know, I think that is reason why the cheaper guns are overgassed. Less problems with cheap ammo. Most sales are from the cheaper guns, so why not make them reliably shoot the cheaper ammo.
Sounds smart and reasonable when you put it that way don't it?
I measure mine with the shank of a number size or fractional drill bit. I then verify the diameter of the shank with a micrometer.
Not as exacting as some methods, but in my experience .001 either way does not make much of a difference.
wolf_walker
09-01-11, 23:11
Would not the actual diameter of the bore effect required gas port size as well? There is variance, hence the slugging of barrels to get a perfect cast bullet size. There is one of the newer guns I vaguely recall, though I think it was a piston setup, that was supposed to have a tapered bore or some such, I'd think that would throw pressure's off no? If so one would need to factor in the bore diameter to really compare gas port diameter overly much it seems to my layman self.
Would not the actual diameter of the bore effect required gas port size as well? There is variance, hence the slugging of barrels to get a perfect cast bullet size. There is one of the newer guns I vaguely recall, though I think it was a piston setup, that was supposed to have a tapered bore or some such, I'd think that would throw pressure's off no? If so one would need to factor in the bore diameter to really compare gas port diameter overly much it seems to my layman self.
OK, now I vote we're getting into minutia here ! ;)
wolf_walker
09-02-11, 08:10
OK, now I vote we're getting into minutia here ! ;)
True enough. I foresee though, a post of "the gas port is tiny on this, but it still runs fine with an H-52 buffer and a spring out of a 85 chevy citation front strut in the buffer, why11!!111.
I made some adjustments to the stick and went to the range Monday.
16" middy
FA carrier
XM193 and PPU75
ETA: IMI 193
A5 buffer
Tested one size smaller insertable gas port.
Dropped down to .073"
This configuration ejected very consistently 3-3:30 with both loads.
Verified lockback on 15-20 mags loaded with single rounds.
I also tested two other buffer combos in the A5 tube.
H2 + CAR spring + .750" spacer
CAR buffer + CAR spring + .750" spacer
Remarkably, the lighter buffers only moved ejection forward slightly, 3:00 and 2:30.
I believe this level of gassing is near ideal, allowing a wide range of loads, buffers and springs to work well.
These insertable gas ports work so well to properly fix overgassed barrels that I'm considering selling them.
OK, now I vote we're getting into minutia here ! ;)
well, this is a technical discussion forum after all. isn't that what it's here for?
the fear of someone reading this and obsessing over minutia is their problem...
It's not uncommon in testing to have ejection in the "correct" range, the bolt to lock to the rear and STILL have a cyclic rate I consider too low for reliability in adverse conditions.
It's not uncommon in testing to have ejection in the "correct" range, the bolt to lock to the rear and STILL have a cyclic rate I consider too low for reliability in adverse conditions.
Agreed. Cyclic rate testing must be the more accurate method to determine gassing.
Been meaning to ask you about that.
Colt advertises the cyclic rate of the M4 (http://www.colt.com/law/m4.asp) and M16 (http://www.colt.com/mil/M16_2.asp) at 700-950, while the M4 Commando (http://www.colt.com/mil/M4Com_2.asp) is listed as 700-1000.
Do you have your own target range(s) for factory new?
Does these change based on total buffer weight and gas system length too?
Seems to me if you are mechanically inclined you already have a fractional and numbered drill bit set. That should get you pretty close. Then just order the gauges between the one that fits and the one that doesn't. Shouldn't be much more than $10. Does it matter a couple of thousandths? Then what are you going to do with the information? Do you really know what size the port should be? I don't really recall a topic/thread that addressed the issue.
Tested one size smaller insertable gas port.
Dropped down to .073"
These insertable gas ports work so well to properly fix overgassed barrels that I'm considering selling them.
Photos or it doesn't exist...
Photos or it doesn't exist...
+1 on that.
John_Burns
09-08-11, 23:56
You should be able to get + or - .001 with a good caliper. A good digital caliper is also a lot more useful for other things than a range of gauge pins.
Nothing wrong with a set of gauge pins but if you are on a budget the calipers will be the better buy based on utility but one has to wonder what most will do if their port is “Oversize”. Live with it is my guess but I am interested in any data other are willing to share.
Plus or minus .001 will have absolutely no effect on function so this level of measurement resolution is more than sufficient. I just measured 2 carbine length gas ports and the range was from .068 to .078 so you can see the range that might be encountered. I suspect some of the grossly overgassed would even exceed the 0.080 in the 7 inch gas systems.
Here is my data from the barrels laying in the shop:
1990s vintage Bushmaster 11.5 barrel with 7 inch gas and 0.750 seat was 0.078 and the gun ran fine from what I remember and was not particularly hard on brass.
YHM 16 inch barrel with 7 inch gas and 0.750 seat with 0.068 port diameter and ran fine but I used an adjustable block to throttle is back a bit. Wide open the .068 port was reliable with xm193 and Wolf but hard on brass in temps from 30 degs to 100 degs at 5000 ft altitude and run wide open.
Just guessing here as I never shot that barrel at really cold temps(-20 F or lower) but .223 pressure loads would most likely work well down to some pretty low temps with the 0.068 port diameter and it might take a serious cold snap to stop xm-193.
Magic_Salad0892
09-09-11, 00:57
Tested one size smaller insertable gas port.
Dropped down to .073"
... Do want.
For us of small minds...
Which manufacturers of carbines and uppers provide 'PROPER' gas port diameters for their carbine length gas systems???
Photos or it doesn't exist...
... Do want.
This has been tested with a low pro gas block but not a FSB yet.
This may also be usefull for a dedicated suppressed setup made with a standard barrel.
Magic_Salad0892
10-11-11, 12:29
This has been tested with a low pro gas block but not a FSB yet.
Even better.
Very informative thread!
I may have missed it but is there an "ideal" gas port size for a 16" mid-length barrel?
It was mentioned in this thread that Colt carbines have a gas port size of between .062 and .064. Does anyone have the gas port sizes on a Colt 20" barrel?
I would also be interested to know if there a difference between the gas port sizes Colt's 14.5 and 16"carbine barrels.
Eurodriver
10-11-11, 20:06
For us of small minds...
Which manufacturers of carbines and uppers provide 'PROPER' gas port diameters for their carbine length gas systems???
Colt, LMT, BCM, Noveske (they don't make 16" carbines anymore, IIRC), DD.
Found what looks to be a reliable data point on the 10.3" Mk18 from
http://en.wikipedia.org/wiki/Close_Quarters_Battle_Receiver
The short 10.3 in (262 mm) barrel length requires special modifications to reliably function. The gas port is opened from 0.062 to 0.070 in (0.16 to 0.18 mm). A one-piece McFarland gas ring replaces the three-piece gas ring set. The standard four-coil extractor spring is replaced with a commercial off-the-shelf (COTS) five-coil spring. An O-ring surrounds the extractor spring. The standard M4 flash hider has been replaced with the M4QD flash hider for suppressor compatibility.
This seems to match closely with Robb's info here>
https://www.m4carbine.net/showthread.php?t=74292
For us of small minds...
Which manufacturers of carbines and uppers provide 'PROPER' gas port diameters for their carbine length gas systems???
Colt
BCM
Noveske
DD
LMT
KAC
There maybe others, but the above are the low fruit.
C4
Relative my earlier posts, I'm thinking or ordering this set (http://www.amazon.com/061-250-Minus-Tolerance-Pin-Gage/dp/B0007Q74XI/ref=pd_bxgy_hi_img_b).
According to dealer:
Increments of .001"
total of ~190 pins
Relative my earlier posts, I'm thinking or ordering this set (http://www.amazon.com/061-250-Minus-Tolerance-Pin-Gage/dp/B0007Q74XI/ref=pd_bxgy_hi_img_b).
According to dealer:
Increments of .001"
total of ~190 pins
Unless you have a bunch of other things that you want to gauge, I would just focus on diameter specifications of the holes you want to measure, and order just under to ~.010" over, in appropriate steps.
If limited to ARs, I can think of gas ports and FCG hole diameters maybe, but not much else. There are places where you can order just specific, individual guages at ~$2-2.50/pin, and you'll come out $$ ahead. Otherwise, spend the $60, get a complete set like you're looking at, and then realize the bulk of those pins will never be used, unless you have other measuring requirements for non-ARs. Just another option to consider.
http://www.auto-met.com/gage/pin_gage.htm
I started looking at ordering individual pins but the various vendors were non-responsive to getting assistance. The fact that these people answered quickly means something to me.
I agree with you that I'd rather have the purpose-assembled set, but the vendors that are capable of such appear to be a giant pain in the ass.
I started looking at ordering individual pins but the various vendors were non-responsive to getting assistance. The fact that these people answered quickly means something to me.
I agree with you that I'd rather have the purpose-assembled set, but the vendors that are capable of such appear to be a giant pain in the ass.
I requested a quote from that link weeks ago and got nothing.
I started looking at ordering individual pins but the various vendors were non-responsive to getting assistance. The fact that these people answered quickly means something to me.
I agree with you that I'd rather have the purpose-assembled set, but the vendors that are capable of such appear to be a giant pain in the ass.
I requested a quote from that link weeks ago and got nothing.
Travers Tool (http://www.travers.com/skulist.asp?RequestData=CA_Search&navPath=All+Products%2F%2F%2F%2FUserSearch1%3Dpin+gage&q=block+id+118248+and+class+level3+id+29694&minPrice=$2.25)
Travers Tool (http://www.travers.com/skulist.asp?RequestData=CA_Search&navPath=All+Products%2F%2F%2F%2FUserSearch1%3Dpin+gage&q=block+id+118248+and+class+level3+id+29694&minPrice=$2.25)
Excellent
Looking at the
(Size Range = '.0115" thru .2865" MINUS')
page if I order every pin listed from 0.050 through 0.080 I wind up with $75. Thoughts on increments?
Excellent
Looking at the
(Size Range = '.0115" thru .2865" MINUS')
page if I order every pin listed from 0.050 through 0.080 I wind up with $75. Thoughts on increments?
The set you were looking at for $59 was suspect. I think they claimed tolerance within 2 tenths. I can't swear that the Travers gage pins are a higher quality but they are made in the USA. Looking at the page I linked to I can see the most sizes you may want are missing (0.062 as an example). Here is another source (http://www.mcmaster.com/#23055a002/=ehq66z) that has a better selection of sizes.
Look at Part No. 23055A002 and then select the pin size on the left column from the drop down menu.
The set you were looking at for $59 was suspect. I think they claimed tolerance within 2 tenths. I can't swear that the Travers gage pins are a higher quality but they are made in the USA. Looking at the page I linked to I can see the most sizes you may want are missing (0.062 as an example). Here is another source (http://www.mcmaster.com/#23055a002/=ehq66z) that has a better selection of sizes.
Look at Part No. 23055A002 and then select the pin size on the left column from the drop down menu.
that works.
Added 0.050 through 0.080 in 0.001 increments. $93, but like you I have more confidence in these. chose "go" not "no go".
Excellent
Looking at the
(Size Range = '.0115" thru .2865" MINUS')
page if I order every pin listed from 0.050 through 0.080 I wind up with $75. Thoughts on increments?
My thoughts are to get them in .001" increments. That way if, for example, a .070" gauge is a loose fit and the .071" gauge is too large, the port diameter is > .070" but < .071".
If that kind of accuracy isn't needed, you could go with .002" increments. Then, if the .070 gauge is loose and the .072" gauge is too large, you'll know the gas port is be > .070 but < .072.
If wanting to save some money you could base your choices around the commonly used port sizes, if known. If the common sizes are .063" (1/16" drill size), .071" (#50 drill size), .081" (#46 drill size) and .094" (3/32" drill size), you could get pin gauges of say, .062", .063", .064", .070", .071", .072", .079", .080", .093", .094" & .095 to start
That would let you measure four different port sizes with twelve pin gauges for $36 if they are $3 each. Gauges can be added to the set as needed.
Note: The numbers I chose are at best, a guesstimate based on reading various posts online. I chose drill bits sizes and pin gauges based on hole size claims. The pin gauge sizes I used may not be the exact pin gauge size available
PS- Here is the link to the drill chart, in case you find it's useful- http://www.carbidedepot.com/formulas-drillsize.htm
that might be a good starting point. I also want to get an idea of wear as well, possibly.
I started looking at ordering individual pins but the various vendors were non-responsive to getting assistance. The fact that these people answered quickly means something to me.
I agree with you that I'd rather have the purpose-assembled set, but the vendors that are capable of such appear to be a giant pain in the ass.
I requested a quote from that link weeks ago and got nothing.
Sorry about the bad info. Did you try calling them?
I've never used them myself, since I haven't made the commitment to building my own and investing in all the tools needed, but when I was thinking about going that route, I began researching gauge pin vendors and that's the one I saved into Favorites for later recall.
The notion of just buying what would be used seemed much more attractive than buying complete sets of never-needed gauges. I just couldn't see investing in wrenches, blocks/clamps, torque wrenches, staking tools, etc. to maybe build one or two weapons. The only way I'd ever go that route would not be for economy, but for the experience and gaining the added knowledge of having done it myself. Iif I ever do make that commitment, that's when I'll reach out to vendors and see what's available.
yeah the point here will not b so much about building things on my own as having the tools available to check things that come across my bench here and there.
You should be able to measure just about any normal AR gas port with this set.
Sorry, clicked on the wrong one the first time.
http://www.mcmaster.com/#2326a11/=ei3xfp
You should be able to measure just about any normal AR gas port with this set.
http://www.mcmaster.com/#2326a11/=ei3xfp
Higher up the page there's a section for ordering individual gauges. I'd go that route instead of getting a whole set. The highlighted set would be minus for a gas port, it would probably cover FCG pins, but would top out just under gas port diameters.
I ordered 31 of the McMaster pins, individually. 0.060 through 0.080 in 0.001 increments.
If wanting to save some money you could base your choices around the commonly used port sizes, if known. If the common sizes are .063" (1/16" drill size), .071" (#50 drill size), .081" (#46 drill size) and .094" (3/32" drill size),
What applications is the .081 for?
.063- 14.5" car
.071- 10.5" car
.081"-???
.093- 20" rifle
What applications is the .081 for?
.063- 14.5" car
.071- 10.5" car
.081"-???
.093- 20" rifle
I do not claim these sizes are actually used, just numbers I've seen tossed about. I have no personal knowledge as to port sizes. These numbers were used only as an example to explain what pin sizes I might choose to measure them. As I stated earlier
Note: The numbers I chose are at best, a guesstimate based on reading various posts online
I ordered 31 of the McMaster pins, individually. 0.060 through 0.080 in 0.001 increments.
You will be good through most mid length gas ports with those sizes.
You will be good through most mid length gas ports with those sizes.
Cool, I'm sure that's most of what I'll encounter. Assume rifle-length would be on the bigger side?
from the 0.080"s to the 0.100"s.
Yes, rifle length will have bigger ports than .080"
from the 0.080"s to the 0.100"s.
Yes, rifle length will have bigger ports than .080"
Nominally .093" for 20" Colt rifle, correct?
A #48 drill bit fits nice and snug in my new barrel.
http://i2.photobucket.com/albums/y37/csch1971/photobucket-14098-1328912941165.jpg
A #48 drill bit fits nice and snug in my new barrel.
http://i2.photobucket.com/albums/y37/csch1971/photobucket-14098-1328912941165.jpg
Looks like the #48 is .0760".
See the link below for drill bit sizes.
http://www.carbidedepot.com/formulas-drillsize.htm
I just removed the FSB off my BCM. I used the same drill bit #48 which is .0760" fits just like it did in the DD barrel. Sorry crappy pic.
http://i2.photobucket.com/albums/y37/csch1971/photobucket-44125-1328929824968.jpg
I ordered 31 of the McMaster pins, individually. 0.060 through 0.080 in 0.001 increments.
Rob, are you satisfied with this selection? I assume you ordered 21 instead of 31?
Cesiumsponge
02-10-12, 22:38
Folks, unless you are using gage pins for a living, an import brand is "good enough". They're to size and simply don't have NIST traceability and typically class ZZ. Gold standard would be a set made by Vermont or Meyer, or Starrett. If you have a bottomless wallet, buy some Deltronic class X in 0.0001" plus and minus increments.
These barrel manufacturers in all likelihood are using reamers in some 0.001" increment. If you're going to start fretting over pin brands, better ditch the import calipers and micrometers I see in photos.
Be careful not to bottom out the pins when inserting into the hole. You risk dinging up the lands on the opposite side of the bore.
Rob, are you satisfied with this selection? I assume you ordered 21 instead of 31?
I wound up with pins, I think, 0.060 through 0.100 in 0.001 increments.
Were I doing it over again I'd probably just pony up for this set, which comes with a box.
http://www.mcmaster.com/#catalog/118/2289/=g782cs
A #48 drill bit fits nice and snug in my new barrel.
http://i2.photobucket.com/albums/y37/csch1971/photobucket-14098-1328912941165.jpg
That's off a tiny bit from what DD says their port size is for that barrel but probably not enough to matter. I can't help but wonder, however, what it would gauge at vs. just sticking a drill bit in.
That's off a tiny bit from what DD says their port size is for that barrel but probably not enough to matter. I can't help but wonder, however, what it would gauge at vs. just sticking a drill bit in.
One thing I've been wondering is how are these gas ports made if there are no drill bits the actual size of the port?
I hate to imagine that bubba gunsmith is just using a smaller one and wiggling it around to open up the hole to the needed size.:eek:
ETA - Okay I just read the post above by Cesiumsponge regarding the incremental reamer. That makes sense to me.
One thing I've been wondering is how are these gas ports made if there are no drill bits the actual size of the port?
I hate to imagine that bubba gunsmith is just using a smaller one and wiggling it around to open up the hole to the needed size.:eek:
I have no doubt that an outfit the size of DD or Colt can order a reamer in the specific size that they need, and that the army of engineers at Colt know how to calculate what they will need prior to ordering.
I suspect that for most, however, it's a matter of trial and error. Which is fine, depending on just how much trial to reduce the amount of error. ;)
The drill felt the same in the both barrels. Maybe its off a .001" or two. The DD barrel gas port has more a chamfer. The BCM is more like a straight 90 degree if that makes sense. I guess to be exact I should had some pin gauges. Anyway my BCM 14.5 middy shoots real smooth and I expect the DD shoot smooth and maybe more accurate since it CHF.
EzGoingKev
02-11-12, 11:49
I have no doubt that an outfit the size of DD or Colt can order a reamer in the specific size that they need
Anybody can custom order reamers or drill bits in the specific size(s) they need.
Anybody can custom order reamers or drill bits in the specific size(s) they need.
That's true, but it's more cost effective to use existing tooling. There are drill bit sizes that run in fractions, letter sizes, number sizes and metric. Then there are core drills which come in sizes standard drills do not. Then there are reamers. They also come in sizes not available in drill sizes or core drills. Between the three types, many diamters are available without ordering custom reamers.
When it comes to drilling close tolerance holes, there are problems with staying in tolerance and staying in control. Depending on drilling method and tool quality, the actual hole can be larger or smaller than the tooling. However, I doubt the tolerances of the gas port holes are very tight
Cesiumsponge
02-12-12, 13:59
Carbide reamers tend to do pretty good cutting to size, but that isn't any guarantee. The way metal machines varies from batch to batch, even if its the same specified, certified material and the same specified heat treat. Metal is funny that way. I suppose the closest analog I can provide is variation between different ammo lots, even if its high grade match ammo.
Special tooling can be expensive...very expensive. likewise, special tooling will tend to vary between batches much more than standard production tooling.
I recently got a the above barrel for g/fs lightweight build. I drove out the tapered fsb pins and used a #52 drill bit which is. 0635 to measure the gas port and it fit nice snug. Anyone measure a Bushy L/W gas port?And would this gas port size be considered over gassed?Thanks
However, I doubt the tolerances of the gas port holes are very tight
The barrel for the M16A1 calls 0.092" + 0.001".
And would this gas port size be considered over gassed?
Does it behave like it's overgassed?
I have not fired round of it yet. I just read on m4c that bushmasters are over gased. I was just wanting to know if anyone has ever measured a 16" l/w with a carbine length gas system. I'm going run a H buffer.
It's usually best to shoot it then decide if it needs to be modified.
Nice timing...I was rummaging through the parts bin to put together a rifle on an FN CHF M4-ish profile 16" car gas barrel I've had laying around, and on closer inspection, the gas port looked HUGE...like enormous. I did a double take. It accepts a 3/32" drill bit, snug...so that's .094", give or take. :blink:
It's like someone drilled a carbine barrel with a rifle gas reamer, and was sloppy about it. Not sure what to do with this thing now. I grabbed this thing in some horse trading a long while back and never even messed with it. I'm half tempted to put it together just to see how bad it will be--but only half tempted.
Anybody ever see anything this far on the large side?
I would put it together and shoot it. When you say "CAR" I hear 11.5" with 5" faux suppressor. Am I correct?
I would put it together and shoot it. When you say "CAR" I hear 11.5" with 5" faux suppressor. Am I correct?
No, '16" car gas barrel' was me being lazy instead of typing '16" barrel for carbine length gas system'...so it's a .094" gas port located 8 3/8" from the muzzle of a 16" barrel set up for carbine length gas with a .750" dia at the block.
Yeah, that's a pretty hot gas port...
I've seen a couple of places online which offer a service whereas they insert a special bushing to close it off.
But I don't know if its worth it unless its some special barrel and/or caliber/type you can't buy again. The way FN churns those things out its easy to find a replacement.
0.086 is the largest I've measured, and that was a 10.3" LW barrel.
that's a shame, it wouldn't be a horrific size for a 11.5". Still there's enough art left in gas port sizes that you might be OK. If not, how are you for SBRs?
Maybe I'll put it together with an H3 and blue SpringCo and see what it looks like cycling on high speed camera. If it's too crazy, I'll pull it and throw it on the lathe.
That's insane big, though. I may just grab another barrel.
I for one wish you would try it and post the video so we can all learn from it.
I for one wish you would try it and post the video so we can all learn from it.
Well, I said "what the heck" and put the barrel on last night. I have a hard time believing that this will work out to any degree of satisfaction, but it might be interesting. I have a strong feeling I'll be changing this barrel at some point soon after the build, but I'll have some data when it's over.
If it doesn't and you don't SBR it you still have some options.
AR15barrels
08-03-12, 12:50
While I am generally opposed to the amount of attention paid to this level of minutiae, it strikes me that if we're going to discuss it so often we should determine how people are making their measurements and assumptions.
I like minutiae.
I use gauge pins in 0.001" steps.
My pins are minus pins, not that it matters if we are only measuring to the nearest 0.001"...
http://ar15barrels.com/tech/colt-m4.jpg
I have documented the profile, weight and gas port dimensions on well over 1000 barrels.
In the case where I find different port sizes, I log them.
In the case where I find the same dimension, I log a couple and then quit logging them.
You learn a lot when you gather a lot of information like this...
http://ar15barrels.com/gfx/drawings.jpg
You learn a lot when you gather a lot of information like this...
yep, you learn a lot of shit that is utterly inconsequential and has zero bearing on anything for 99% of shooters, let alone the 99% of purposeless gun owners.
AR15barrels
08-03-12, 14:01
yep, you learn a lot of shit that is utterly inconsequential and has zero bearing on anything for 99% of shooters, let alone the 99% of purposeless gun owners.
Even though it has no bearing, it's still good to know who is more consistent, and who have crappy QC...
Even though it has no bearing, it's still good to know who is more consistent, and who have crappy QC...
and that's just it. This thread was started months ago in reaction to a bunch of people posting "what's my gas port size" threads. In the absence of any knowledge as to what it should be, or anything else to compare it to, and with no idea what they were going to do with that information once they had it, there wasn't anything for them to do with it. They may as well know what kind of batteries the smoke detectors take on the International Space Station.
AR15barrels
08-03-12, 17:41
and that's just it. This thread was started months ago in reaction to a bunch of people posting "what's my gas port size" threads. In the absence of any knowledge as to what it should be, or anything else to compare it to, and with no idea what they were going to do with that information once they had it, there wasn't anything for them to do with it. They may as well know what kind of batteries the smoke detectors take on the International Space Station.
I always get requests for gas port info.
People don't realize the effort that I have into developing gas port sizes for all manner of different combinations of bore sizes, chamberings, barrel lengths, gas system lengths etc...
I have a pretty wicked excel sheet going.
I'm not gonna just freely give that work away to anyone just for asking though.
If they have a problem and ask nicely, I will generally point them in the right direction, or offer to fix the problem, but 9 times out of 10, it's not even the gas port size...
I have a hard time believing that this will work out to any degree of satisfaction, but it might be interesting. I have a strong feeling I'll be changing this barrel at some point soon after the build, but I'll have some data when it's over.
Yes, .093 is very big for even a 16" mid gas with heavy spring and buffer. Never mind a CAR gas.
An insertable gas port is probably the best way to make that barrel work properly.
Well, I said "what the heck" and put the barrel on last night. I have a hard time believing that this will work out to any degree of satisfaction, but it might be interesting. I have a strong feeling I'll be changing this barrel at some point soon after the build, but I'll have some data when it's over.
Any updates on this Duane?
Did you ever shoot this barrel with a ridiculously large gas port?
patrick sweeney
04-12-13, 14:09
but 9 times out of 10, it's not even the gas port size...
Words that should be carved over every bench used by visitors to this forum.
Yes, .093 is very big for even a 16" mid gas with heavy spring and buffer. Never mind a CAR gas.
An insertable gas port is probably the best way to make that barrel work properly.
Holy smokes! My 14.5" Middy runs about perfect with an .080"... my other 14.5 middy is a little fussy with an .076".
.093" might be ok on a 16" with Rifle length gas, but Wow... not much else.
AR15barrels
04-12-13, 15:42
.093" might be ok on a 16" with Rifle length gas, but Wow... not much else.
0.093" is perfect on a 20" rifle gassed bull barrel.
0.093" will be too small for a 16" rifle gassed barrel unless it is a 17 Remington, 20 tactical or 204 Ruger.
What increase "decrease" is needed to make an actual change in the gas pressure? It seems you aren't going to increase the hole a thousandth then check operation?
AR15barrels
04-12-13, 18:53
Holy smokes! My 14.5" Middy runs about perfect with an .080"... my other 14.5 middy is a little fussy with an .076".
.093" might be ok on a 16" with Rifle length gas, but Wow... not much else.
What increase "decrease" is needed to make an actual change in the gas pressure? It seems you aren't going to increase the hole a thousandth then check operation?
You can see the function change with 0.003" gas port size changes if you know what to look for.
Thanks that is a tiny increase . What is the increase due to gas erosion after X amount of rounds fires? I would imagine the rate of erosion has to vary depending on many factors but a ball park figure?
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