I’ve been doing a lot of research in anticipation of my next purchase, a semi-auto sniper rifle. I am, however, stuck on one major issue… Gas piston or direct impingement? I’m a big fan of gas piston systems, but I can’t help but think that having a couple of ounces of reciprocating mass on top of the barrel is going to have a detrimental effect on down-range accuracy… Especially when the barrel is hot. I’m thinking that a gas piston is great for carbine length rifles, but direct impingement might be better for the sub m.o.a. accuracy I’m looking for. I’m not asking for opinions on which brand is best, just thoughts on gas piston vs: DI for long range accuracy.
Gas. I believe the AR owes its accuracy to its tight tolerances and lack of an operating rod. The less parts means less to vibrate. The piston system will keep the gun cleaner and keep heat out of the action which is a more beneficial in a CQ rifle than a sniper rifle. From research Ive done on this subject, Ive found that the GP systems are not as accurate as the GI systems, so for a sniper rifle, Id definitely go gas.
I did quite a bit of research on the subject of Gas Piston systems in AR’s before I built my 6.5 Grendel. My conclusion was that these systems, no matter which you choose, are still an immature technology though some recent additions to what is available have moved the effectiveness and quality up several notches.
Is is worth an extra $350-500? Not in my opinion, at least not yet.
Sabre Defense has just introduced a Gas Piston equipped rifle and complete upper in 5.56 only, it remains to be seen how that will work for them.
I went with Direct Impingement; lighter, simpler, proven.
I have a gas piston M-4 that I love, but I don’t expect sub m.o.a. performance out of it… I forgot to mention I’m going with .308 not 5.56, so that limits my options. I’m leaning towards the actual LRT SASS from DPMS, which is DI. In my mind, I just can’t see getting reliable sustained accuracy with a gas piston banging back and forth on my barrel.
The FAL has functioned well with a gas piston for nearly six decades, the accuracy limiter on the FAL is the trigger system. Also, the M14/M1A is a very accurate gas piston rifle. I don’t think anyone has proved the gas piston idea to be inherently less accurate than direct impingement, it MAY be on AR form factor rifle, or it may not.
Perhaps the AR10 form factor isn’t amenable to a gas piston system, but it should be.
Do a google search for “ar piston gas accuracy” and youll find several reviews and even threads from other sites talking about decreased accuracy in the AR.
I guess the real question is, do you want to stick with the AR platform? If so, I would stick with a DI system which is a well matured design. As Fastpat has said, piston ARs are relatively new and in carbine classes some of the designs are encountering some problems. The manual of arms of the DI system is pretty well understood and if you follow standard cleaning and lubing protocols, you’ll likely be in better shape than going with the latest and greatest piston system.
Other advantages of the DI system are:
-Reduced overall weight
-Reduced forend weight(keeps the balance right)
-Simplicity of design: This makes for easier and cheaper manufacturing, as well as making it easier to maintain good quality controls. The more complex the design the harder it is to keep the good QC.
-Reduced barrel vibration: This seems to be important in the AR platform in keeping excellent accuracy.
Obviously a DI system gets more carbon buildup in the receiver and bolt assembly, however if you are keeping it well oiled you can keep a top tier rifle running reliably for well over 1000rds(some can go 2-3x that). Then you go clean it as the manual tells you to. It’s really not a very difficult cleaning routine.
I say go with a DI system and save yourself from the stress of getting a system that may or may not be perfected.
I was thinking that DI was the way to go, it’s always good to get some well informed feedback though.
As a gas piston AR owner, who bought it for CQB - if you want to punch paper, gongs, gophers, etc., with your ‘urban forward observation’ rifle (we don’t use the “s” word on boards, unless you have a .mil/.gov address! ;)), go DI.
Flexibility - I love the AR platform - and although I grew up on an A1, DI, 24 years ago, but I really like piston uppers these days(couple of years or so). I personnaly think there is a little more shove in GP over DI, and I see the sweet spot in FA, and suppressed - 10.5" barrel, etc for GP. These threads alway go either “if stoner meant it to happen he would have done it that way” or “DI works every time for me so why would I change what works” or “if it aint broke…” People have issues with change, always.
Now in a .308 platform…other then the venerable FAL of M14 system, I have only done DI(Armalite) Who can argue those are not effective? I am very interested in how the SCAR heavy or LWRC REPR works out. There are some bugs in the AR10 system that just have not been worked out and have not been convinced the DPMS system has worked out…
Back to the original post, does anyone have any data on this? All I could find were people saying that piston is less accurate. Im curious if any comparison tests have been done at longer ranges and what they concluded. Id like to know just how less accurate these piston AR’s are.
Gas piston will have an effect on accuracy.
https://www.m4carbine.net/showthread.php?t=32479
Also, since the purpose of your rifle is long range/accuracy, have you considered the 6.5 Grendel from Alexander Arms? If I was after a sniper rifle built on the AR platform, thats the route Id go. Heres a link for ya. 
You want a DI gun for your stated application.
It depends on if he actually likes to shoot his rifle. Considering that the cheapest practice ammo for the Grendel is Wolf(which is rarely available) runs for $.75 a shot, he won’t get a lot of shooting time in. The Wolf stuff is very inaccurate for shooting at long distances. If he actually wants to make tight groups at long range he’ll need to invest in ammo that runs closer to $1.50-$2.00 a shot, or setup for reloading and pay 3x the price for Grendel components. If he wanted to go odd-ball caliber route, he can get a quality load in 6.8 SPC for long range shooting for closer to $1 a shot.
I think he should stick with .308 personally. He has a much wider array of ammo and accessories to choose from, and for cheap training time he can get brass cased .308 for as cheap as $.30 a shot. A .308 is a better investiment and with quality heavy bullets the .308 can shoot rings around the Grendel at long range.
sgalbra76: You make some good points. Sticking with 308 makes more since than sticking with gas vs piston. Although on paper, the 6.5 outperforms the 308, its just not practical unless you make the $$$big bucks$$$ and can afford to feed it.
I wouldnt bother with a piston on a 16" gun. Total waste of money.
On SBR’s, yeah, I think pistons have an advantage.
My LMT 10.5" was ammo picky. My LWRC 10.5" will eat anything
Most of the actual issue “sniper” semi autos are at least .30 caliber and gas piston. The question has been answered, issued, and used for over 30 years.
A “sniper” in intermediate calibers is really a misuse of the design purpose. It’s a 600 meter assault rifle class, not a precision long range shooter. However, it can do the job - properly equipped - and on paper targets holds all the records over .30 piston designs.
If shooting paper, go DI on the AR platform. In the field on living targets, .30 gas piston. The users have made the distinction pretty clear.
imo,
I’d go with a DI for that application.
Jet engines don’t use pistons either…
agree on both counts.