Bolt Carrier exhaust vent size variations

[markm]

On of our members here was talking to me about the idea of an experiment where the exhaust ports would be welded shut. ??

[tom12.7]

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.

[markm]

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]

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.

[Clint]

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.

[bowietx]

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.

[tom12.7]

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]

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.

[markm]

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.

[tom12.7]

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?