AR Sponsor
Posted: 10/21/2005 4:47:26 PM EDT
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I have been doing a bunch of reading on the mechanics of AR operation, and I'm trying to understand a few things about gas block position on the barrel. For the shorter barrels, obviously the front sight / gas block has to be located closer to the breech. This results in higher pressure since the gas has had less time to expand at this point in the bullet's travel through the barrel. Here's where I'm a little unsure... From what I understand, the current solution to compensate for these higher pressures is to make the port in the barrel a smaller diameter so that a smaller volume of gas will enter the tube. Am I correct in that assumption? I also read that the smaller port is much more susceptible to erosion which contributes to a shorter useful life of the barrel. My question is, couldn't the port remain the same size (to prevent early port erosion) and a vent be placed in the gas block instead? This vent could be adjustable to release excess pressure to the atmosphere with a set screw or something, to aid in timing your action. Obviously, this vent would seriously reduce the effectiveness of any supressors that might be installed, because some of the gas is being vented out, and there could possibly be some flash involved as well. Does anyone have any more info on how the shorter barreled AR's were engineered? |
Poorly... The short barreled family came about simply because a shorter rifle was wanted/needed. The evolution of the rifle had the barrel length increased bit-by-bit to the 11.5" version and the moderator added to increase back pressure and cut down noise -- even at this, it was still had reliability problems. You are correct that the pressures are higher at the shorter gas port location; nearly twice as high at 24,000+ PSI, the odd thing is that do to reduced dwell time (the time that the pressure is high, determined by the length of the barrel after the port), these weapons generally have the gas port opened to a larger size to run... not the best answer of course. The axiom of this voodoo is that opening the gas port of the AR is a band-aid fix for the port being in the wrong place. What is more important than gas port pressures is the internal bolt pressure, ideally you want to run about 1000 PSI peak in the bolt (the M4 runs about 1600). As for errosion, the gas ports tend to flame cut up into the barrel at about a 45 degree angle, forward from the port hole in the bore -- hard to see unless you bifurcate the barrel. The shorter systems are naturally worse at this. As for external venting... the system does not play well with the idea. This is fluid dynamics and the gas follows the path of least resistance; adjustable (bot manual and automatic) gas port devices have been tried with varying succes -- Check out some high speed video of the M4 firing full auto (especially with a can!) and you will see gas spewing out like you can not imagine... the running joke is that it is a good thing we only have 30 round mags, as the gun would not run much past that... The mid-length gas system on a 16" barrel is very smooth and reliable, a truely nice setup. We made a alternate location gas port M4 (some on here have seen it) dubbed the M4Ward, this seemed to tame the gun nicely -- but the individual modification cost was prohibitive for wide scale adoption (then anyway). Hope this helps a little? |
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