Bolt Carrier exhaust vent size variations

[sinlessorrow]

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.

[tom12.7]

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?

[tom12.7]

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.

[Joe Mamma]

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.

PWS di carrier.jpg

[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]

At work, but this is recent.

LAV FIREClean Lube Test: http://youtu.be/S0OAsOCEJfQ

[tom12.7]

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]

Just checked and my MP4 BCG has vent ports that are .1" in size.

[ABNAK]

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.

[Clint]

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]

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?