Home AR-15 Smyth Busters: Are Piston-Operated AR-15s Better?

Smyth Busters: Are Piston-Operated AR-15s Better?

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Brownells Gun Techs Steve Ostrem and @CalebSavant are back to take on the myth that piston-operated AR-15s are better / more reliable than standard direct-impingement ARs. As the guys demonstrated in an earlier Smyth Busters episode (“Is the AR-15 Really Piston Operated?”), the AR-15 is not actually a pure direct impingement rifle but a kind of hybrid, with the bolt operating like a piston. In this episode, “direct-impingement” means an AR-15 with the traditional gas tube above the barrel.

Caleb points out there have been a lot of attempts at piston-driven AR-15s, some of which were terrible. The Heckler & Koch HK416 was purpose-designed to be piston-operated, and it works VERY well, as does the Brownells BRN-4. Any good piston-operated AR-15 style rifle will be more expensive than a direct impingement one because it just costs more to manufacture the additional moving parts. And it is heavier.

So what’s the benefit of a piston? A piston AR runs CLEANER and COOLER. It doesn’t blast hot gas back into the receiver and bolt-carrier group, which increases the gun’s longevity. This is a huge benefit in full-auto fire, too. A direct impingement AR-15 runs hotter and dirtier, but the rifle is lighter. And after 60+ years of development and refinement, it is also very reliable.

What about those “plug-n-play” piston conversion kits? They involve engineering compromises that cause “bolt tilt.” In direct-impingement, the gas pushes on the bolt in line with the bore so the bolt and carrier move in a straight line. With a piston conversion, the force hits a lug on top of the carrier where the gas key would be. This force is off-center, causing the carrier to tip down at the rear, tilting the bolt upward. The result is accelerated bolt wear and malfunctions. Purpose-built piston guns like the HK416, BRN-4, and AK series are designed to keep the piston force properly centered.

So the myth is BUSTED. A piston AR is not always better than a direct impingement AR-15. Direct impingement is more reliable than a piston conversion kit. But a purposed-designed piston system is more reliable than either of them. A direct impingement AR-15 is the most cost-effective option for sport shooters. But if you have the spondulicks, try the HK416 / BRN-4 route.

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40 COMMENTS

  1. Cleaner, cooler, better for suppressors. Seems like if you actually have to use the 2nd amendment for its intended a piston system would be a better weapon… don't get conversion kits, they are prone to tilt and cause malfunctions.

  2. Piston: more weight, more moving parts, more prone to failure due to more moving parts, can get clogged up and operate at a reduced efficiency, noticeably less accurate.

    Direct impingement: gets a little dirtier in the breach area. Most gun owners are lazy as hell and turn their laziness into bragging rights on how long it is been since they have performed any maintenance on their rifle. That's like bragging how long it's been since you have checked or changed the oil in your car, pure stupidity

  3. There is a company that sells conversion kits (as well as dedicated piston uppers) that has solved the problem with bolt tilt with a new bolt carrier design, and includes the bolt carrier in their conversion kits that are as reliable as the dedicated uppers.

  4. I would like to say a piston ar also has more moving parts. Which in turns means more parts to break. Some of us keep spare parts for our Di guns. Do you keep spare parts for your piston Gun?

  5. DI gets complicated though when you're talking reliability with suppressors. This seems to be the only reason to consider a piston system with adjustable gas system. Otherwise, stick with DI, much lighter, much accurate, much better.

  6. Years ago, I purchased a CMMG piston system from Brownells. I installed it on 11.5 SBR Bushmaster with a LaRue handguard. I drilled and pined the gas block and had to mill the inside of the LaRue so it could fit. I put tens of thousands of rounds through it at action Rifle and three-gun matches. I did purchase Two more of the CMMG systems from Brownells and have had no issue with them (lower round count). I notice a different recoil impulse to add to the differences. My two cents. Keep up the interesting videos.

  7. Holy shit. 1. Hk416 isn’t that great. Its over priced over gassed and is all proprietary. And apparently needs to have its piston cleaned? Lwrc is self cleaning and not completely proprietary. Also is gassed correctly

    2. “It runs cleaner” lolololol put a suppressor on it and see how clean it is.

    3. “It runs cooler” oh my God i used to think you two clowns knew what you were talking about.

    Tldr pistons are shit and an old operating system. Siphoning gas to operate an internal piston is the modern way.

    UNSUBSCRIBED.

  8. Back when I was imagining building a rifle I thought for sure that piston was the way to go. It seemed so obvious! After learning a bit more about how the AR platform runs, and hearing about how the piston-rod-driven bolt carriers sometimes can't take the impact after a while, it became clear what the benefits of having the piston inside the BCG are. You're directly driving the bolt carrier back with the gas expanding inside of it, and it unlocks and yanks the bolt back with it. That's a system that involves much more mass whose inertia must be overcome which means it can run a bit smoother IMO. I'm no expert, these are just the thoughts of a total novice.

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