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Direct gas or piston? AR users argue endlessly over two operating systems. So what's all the fuss about?

WANNA START A FIGHT? NEXT TIME you're at your local range, find a guy who's shooting a traditional direct-gas-impingement AR and another guy shooting a newer gas-piston AR and ask, "Hey, which gas system is best?" Then get out of the way.

Of all the many controversies surrounding AR-platform rifles, none inspires more acrimony--or more close-minded foolishness--than the DGI vs. GP argument. Even among serious AR shooters, the discussion often borders on the absurd. First, to read the debates on many Internet forums, you'd think ARs were the only gas-operated firearms ever designed.


Of course, all semiautos are gas-operated, even "recoil operated" guns like the 1911, since gas is what causes recoil. Second, most of the actual technical performance differences between DGI and GP ARs really emerge only at the extremes of performance demands, "extreme" meaning either a demand for extreme accuracy or a demand for maximum heavy-use reliability without proper maintenance under the most adverse environmental conditions. You can't have both.

This is not to say that there aren't actual differences between the two designs. Proponents of the original DGI AR-15 system argue that it has more than a half-century of proven battle experience behind it, that it is a simpler design having fewer moving parts and fewer nonlinear stress points on the bolt-carrier, that it is therefore inherently more accurate. They claim that it is lighter weight than a GP system gun and that--feature-for-feature--a DGI AR is less expensive than a GP AR. Every one of those statements is absolutely true.

Proponents of GP ARs argue that they do not vent hot gases and firing residue directly into the bolt and chamber area, that they therefore run cooler than DGI guns. They do not stress and wear out bolt-system points so readily in sustained fire and are more reliable when scrupulous maintenance is not an option. Every one of those statements is also absolutely true.


A DGI system does not employ a separate gas cylinder, piston or operating rod assembly. Instead, high-pressure gas is tapped from a nonadjustable port built into the front sight (or gas block) assembly and travels through a gas tube above the barrel directly into a chamber in the bolt carrier behind the bolt itself, pushing the carrier away from the bolt. This eliminates the need for a separate piston and cylinder, thereby saving weight, lowering costs and reducing the operating mass of the parts. It can provide a faster cyclic rate and better mechanical accuracy by keeping all reciprocating masses on the same axis as the bore.



The main disadvantage is that the breech parts become fouled more quickly as vaporized metals, carbon and impurities in the gas condense and build up on the bolt and inside the primary operating mechanism. Closer attention to maintenance is required. In an AR-platform design, the amount of fouling depends primarily on the type of propellant used. A nonspec propellant used in early-generation military 5.56 ammo was largely to blame for the MI6's problematic reputation when originally deployed.

Further, the direct venting of the combustion gases into the AR-15's bolt and bolt carrier makes them really hot really quickly. In rapid, sustained fire, this can alter the temper of metal parts, accelerate wear and decrease the mechanism's service life. Hot gases also dry lubricants rapidly and make the operating parts too hot to handle when clearing malfunctions. Thermal expansion in the action can cause tolerance changes. The faster and harder you have to use the rifle, the more this shows.

With a GP system, none of this happens. The gas used to operate the firearm is isolated far from the breech and contained within the gas cylinder, then vented far away from other working parts, and your hands. You can rip off a 20-round magazine through a gas-piston AR and put your thumb against the closed bolt without pain. Do that with a DGI AR and your flesh will sizzle.



But what about the "extreme accuracy" thing? Pretty simple, actually. Direct gas impingement is better. All GP systems have moving parts operating out toward the front of the barrel, to which they are in one form or another attached. A DGI AR does not. Having moving parts attached to a barrel affects harmonics and introduces a tiny inconsistency into the exact location of the muzzle relative to point of aim with each shot. "Tiny" is the operational word. I've never been able to see it at 100 yards, but in a 1,000-yard match, it matters. So nobody's won Camp Perry (yet) with a GP AR.


Extreme long-range AR varmint shooters don't use GP ARs either. On the other hand, just about every current top-brand GP AR and DGI AR I've reviewed in recent years will shoot MOA groups at 100 yards if I get serious with them--which is all the vast majority of AR users will ever want or need.


The range of makes and models of GP and DGI ARs on the market today is huge. Nearly all major-name manufacturers offer both. Plus there is a wide variety of GP conversion kits available, some simple "drop in" designs, some requiring full barrel system swaps. Not all GP systems are created equal. Some (both on ready-made rifles and on conversion kits) have multiple settings (up to four) to adjust for varied pressure loads or for poorly maintained guns. Others have fixed settings.

The weight issue, incidentally, largely depends on make and model. A 16-inch Stag Arms Model 1 DGI carbine weighs 6.29 pounds; the 16-inch Stag Model 8 GP carbine weighs only .45 pound more. Sig Sauer's 16-inch M400 SRP DGI carbine weighs six pounds even; the 16-inch GP SIG516 Patrol weighs 7.4 pounds. A lot of that is due to the fact that Sig's GP system has more features than Stag's relatively simpler GP system. If you opt for a GP design, and weight matters to you, check out the specs before you buy.

Cost? Oh, yeah. GP ARs are more expensive than comparably featured DGI guns. All of them.

My initial experiences with the AR platform came with early versions of the M16 back in the 1960s and '70s in humid and swampy corners of the world. Some of those versions got a lot of bad press back then for their unreliability (some of it deserved), but I personally had no such problems. I always managed to find the time to maintain them, just as I would any combat rifle. Those who didn't had different experiences. Today's high-stress users find themselves on the opposite end of the environmental scale, operating in hot, dry and extremely dusty places. Based on all reports back from that front, the same equation still applies.

Fact is, most AR users do not keep their guns maintained as well as they should. One result has been the recent eruption of GP ARs and conversion kits.

So, what's a guy to do? My feeling is this: If you need the level of refined match-rifle accuracy necessary to win championships at Camp Perry on the one hand, or want the lightest-weight AR carbine possible on the other, you need a DGI AR. But if your rifle needs to endure heavy, sustained bursts of rapid fire under adverse conditions without regular maintenance, you're better off with a GP AR. For everyone else, it really doesn't make any difference. Buy the one you like.

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Author:Metcalf, Dick
Publication:Guns & Ammo
Date:Aug 1, 2012
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