The gas system: when more isn't better.

Author:Zediker, Glen
Position:Up On ARs
 
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All gas-operated semiautomatic rifles have port pressure as an issue, or let's say it's an issue in designing the basic operating system. Port pressure is not the same as chamber pressure. Port pressure reflects on the amount of burning propellant gas available in the barrel gas port area to enter the gas system and operate the action. An AR-15 has an impulse system, which means the gas goes through a hole in the barrel and into and through a gas tube. The tube is mated with the bolt carrier key. There is a certain amount of gas the system needs and any more is more than it needs. When it gets more than it needs, it's possible for the carrier to start moving too soon and the bolt to unlock too early. A 20" standard-form rifle using commercial ball ammo doesn't exhibit problems. That's because it's how it was built to function. However, since we jack around with (say "engineer" if you're full of yourself) different barrel lengths and bullet weights for use with AR-15s, paying it some attention helps attain optimum function, and, well, happiness.

The gas volume moving through the space it can occupy determines how hard a hit the system delivers. Take a standard setup (20" barrel, 13" port-hole location) and put on a 24" barrel, and there is now a good deal more pressure built up inside the bore. Make it a 26", then a 28", and we're literally cooking with gas. This additional pressure gets into the system. Port pressure increases. Carbines can cook, too. Their ports are too far back.

Shortened (to none, sometimes) brass life is usually the most obvious indication of gas system overload--more gas too soon, worse spent case condition (excessive case head expansion). The extractor tries to yank the case out of the chamber too soon. Other consequences are more tendencies for primer problems (cracks, pierces, blow-outs) and also more battering on associated parts due to increased carrier velocity. This can get slap-outrageous with .308-sized ARs, like the AR-10 and SR-25, and is a huge factor when their native .308 round is necked down to a smaller caliber. A .243 Winchester chambering, for instance, increases port pressure radically: a smaller bore diameter and more, usually slower burning, propellant, means, respectively, less room for gas expansion and a longer pressure curve within the bore. More pressure at the gas port--higher port pressure.

If we can soften the system and delay bolt unlocking without compromising cycling reliability, that's bliss.

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