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Somewhat, but the main thing is the diameter of the gas tap combined with barrel length.

On an AR-15, the standard gas tap is .075-.085 inch. Now, on a 14.5 inch barrel or a 16 inch barrel, that is fine. BUT, on the 11.5 and 10.5, you need a larger diameter tap to allow enough gas to impinge on the bolt before the pressure bleed off due to the bullet exiting the barrel. So, you actually have less pressure on the short barreled guns than you do on the longer barreled guns.

The bad part of all this is that on a full auto rifle, if the tap is opened up too much, you increase the cyclic rate of the gun to an alarming rate. This can cause the life of the gun to be shortened dramatically and it can cause injuries.

In short, the key to pressure is how much barrel is left after the tap...

Mike
 

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That's a simplification of the problem John. It's not the distance between the tap and the chamber that is the issue. It's the gas volume and pressure in that distance that is the issue. The AR15/M16 requires a minimum amount of pressure to operate. There are several ways to measure the actual pressure but the best way of fixing the problem is stepping up the gas port size. Pig tails are okay but they do have some undesirable side effects such as changing the heat dissipation ability of the barrel as well as sometimes requiring a tap in the barrel that is larger than safety suggests should be used. The largest safe tap should be no bigger than .095 and that is pushing it.

The best rate reducers for the AR and M16 family is buffers. You can get rate reducing buffers that work wonderfully without screwing with the design.

Mike
 

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Yeah, let me dig up the info and I'll post here in a bit... there is a super heavy hydrolic buffer that will reduce the cyclic rate from 950 to about 650...

Mike
 
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