The gas system: when more isn't better

Guns Magazine, Nov, 2005 by Glen Zediker

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.

Built-Ins And Bolt-Ons

Answers to unavoidably higher pressures, as with the just-mentioned smaller caliber rounds used in an SR-25 or AR-10, are found in changing the gas system blueprint. Even adding the four additional inches of barrel length commonly applied to most makers' .223 chambered varmint or target rifles makes for excessive port pressure. A good custom builder won't overlook addressing this issue. At the least, port location will move forward (an inch works wonders). Over this new port will go a different gas manifold that's either been extended to accept a standard length gas tube, or the rifle will get a custom length gas tube. Ideally, this new manifold will also be adjustable. There are also accessory gas tubes that either provide adjustment means to regulate gas flow or additional expansion area.

Even if port pressure is not a hurtful problem, or one that much can be done about (as in an NRA Service Rifle or a sto' bo't 24"-barrelled varmint rifle) it is worthwhile to soften the system, in effect. The less rifle movement during firing and action cycling, and the less the effect from "hot" loads, the better. It's even more worthwhile when you can do it yourself, and way more better when it can be done quite simply.

EVOLUTION GUN WORKS INC.

(215) 538-1012

WWW.EGW-GUNS.COM

JP ENTERPRISES INC.

(612) 486-9064

WWW.JPRIFLES.COM

SUPERIOR SHOOTING SYSTEMS INC.

(806) 323-9488

WWW.DAVIDTUBB.COM

BROWNELLS INC.

(641) 623-5401

WWW.BROWNELLS.COM

OLYMPIC ARMS INC.

(360) 459-7940

WWW.OLYARMS.COM

LES BAER CUSTOM INC.

(309) 658-2716

WWW.LESBAER.COM

RELATED ARTICLE: Weight gain isn't always bad.

Two too easy ways to soften action cycling are bolt carrier weight and buffer spring "power." David Tubb developed a carrier weight system called "CWS." It's a hollow metal cone with a flange on one end. This installs into the back of a bolt carrier. The hollow center of the CWS can accept different weight inserts. The reason it works is plain old physics. Anyone who has pushed anything knows that the heavier it is the harder it is to move. The heavier a bolt carrier the harder it is to move, and the effect is that it accelerates slower. The slower it starts back the longer time there is for gas pressure to subside and dissipate. That's great. The extra carrier weight would then seem to be a bad deal when it all comes back home to chamber the next round since extra weight is harder to stop, but since the carrier doesn't have a chance to accelerate to the same speed a standard carrier does, it's actually softer in operation. Win. win.