Bolt Carrier exhaust vent size variations

[tom12.7]

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.

[Clint]

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 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]

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 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.

[VIP3R 237]

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.

[LRRPF52]

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.




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]

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.

[tom12.7]

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]

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

[tom12.7]

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.

[Clint]

Interesting.

I never knew the Dutch AR10s had longer gas keys.

They also have 3 vent holes.

Found a pic.

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).