Piston vs Direct Impingement AR15 TestingOne of the things I have always wondered was how two identical AR15s would run with the only difference being that one is a direct impingement (DI) system and the other, a piston system. For those that are newer to AR15s, the vast majority of AR15s and military M16s run on a gas pressure bleed off port on top of the middle of the barrel which blows hot gas back through a small tube. That pressure blows the carrier back, unlocking and cycling the bolt, and carrier.
With gas piston systems, the process is similar however the gas bleed-off from the barrel pushes a piston and op-rod assemble located inside the gas block. The piston is pushed which moves the attached op rod which in turn pushes on a special bolt carrier and ultimately cycles the bolt. Most AR15 piston systems are either short or long-stroke systems. 
PWS’s is a long stroke system which is nearly identical to typical AK designs but is a proprietary system than cannot be retrofit to any AR15. The Short Stroke retrofit piston systems such as Superlative Arms, used in this test, can be retrofit to nearly any 5.56/.223 AR15. The long stroke piston systems have a stroke that extends the majority of the carrier stroke where the short stroke provide a quick short push to the carrier.DIRECT IMPINGEMENT - The advantage of the DI system is that it is simple, lower cost, and does not require the precision setup for reliable operation that the piston systems does. The assembly precision is not as critical and the gas port alignment and fore-aft gas tube length can be off by a little and everything is still likely going to work just fine. The most significant disadvantage to the DI systems is they blow hot gas and burnt cartridge debris back into the action to power the cycling process which ultimately gums things up a bit and mandates cleaning ever 1500 or so rounds. With the addition of an adjustable gas block to allow tuning of the gas pressure, recoil can be greatly reduced (typically by about a third), less gas pressure, heat and debris is blown back into the action. In theory an adjustable gas block can deliver a \clearer and longer running DI gun. See my other article here on Adjustable Gas Blocks. With the use of suppressors though, gas pressures rise again and excessive gas is blown back into the shooter’s face if they are not re-tuning and lowering the gas block pressure for suppressor use.
PISTON - The main advantage is that piston systems do not blow hot gas or debris back into the action even with the addition of a suppressor. The cleaning interval can be greatly extended to 5000 or more rounds without a significant impact on reliability. Many shooters have noted 7000-10000 round cleaning intervals. My experience with previous builds and article has been that a piston driven gun seems to run so deceptively clean that I tend to forget completely about routine cleaning. Most piston systems also feature adjustable gas blocks which allow tuning of how much gas pressure the op rod piston sees. Piston systems overall have more mass in the system which means they soak up a bit more energy during each cycle. The result is a different recoil impulse… sometimes sharper and sometimes softer than DI systems. That extra mass can be problematic when working with lower pressure calibers such the 300 Blackout and especially with subsonic round. Some have figured out workarounds to increase gas pressure to drive the piston system to get 300 BO to work well in a piston gun. Suppressor shooters ideally want a 300 BO piston gun to have cleaner running gun that does not constantly blow hot gas back in their face. For this test I stuck with .223 builds.
THE PISTON VS DI QUESTION - The interesting question I had in mind was that if you had an optimized DI system with a properly adjusted gas block, how would that compare to a properly tuned piston system. For this test, I built a custom mirror polished Barrett billet lower with a Hiperfire Elite Trigger and Ace UL rifle length stock and buffer. The uppers are both custom naked polished PWS Bootleg uppers with Faxon Match Series- 16" Pencil, .223 Wylde, Mid-Length, 416R, Nitride, 5R, Nickel Teflon Extension, and a .625-inch gas block size. These Match grade Faxon barrels are phenomenally accurate for their skinny super light profiles.Both upper builds used PWS FSC compensators though I did test a number of different compensators with the system to see if there was some difference between certain comps and each system. For the record there was no magic combination of system vs compensator, each comp performed similarly on each respective upper.
The adjustable DI gas block was a Superlative Arms adjustable gas block and the retrofit Piston system for the piston upper was also Superlative Arms. The DI system featured a Faxon Lightweight Bolt Carrier to allow lower gas pressure setting and recoil reduction. The piston system features a Faxon Bolt with a Superlative Arms piston system (gas block, piston, and carrier).
In the scope of this test, the handguard, charging handle and optics really do not matter as long as the weight is not drastically different. In this case, I swapped between the Sig Romeo4T Red Dot, Holosun, and an Eotech XPS on each upper and saw no perceptible difference between the recoil impulse due to the optic weight swaps.
COST
There really is not a big difference in cost assuming you are buying quality DI components. A DI Adjustable gas block retails for around $100 and a quality Bolt Carrier Group is another $150. On the Superlative piston retrofit side of the equation, the complete kit minus bolt is $280. Add in $100 Bolt and the difference between the two systems is only around $130.
TESTING - CLEANLINESS
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| DI Left Piston System Right |
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| DI System |
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| Piston System |
WEIGHT
The piston system is heavier by not by much. The two handguards weighted in at the same weight. Technically the carbon fiber handguard was lighter than the hyperlight aluminum forend, however the Dolos quick barrel detach system added back some weight to the carbon fiber handguard. As tested without optics, the Piston system complete upper was 3-lb 12 oz and the DI complete upper 3-lb 1oz. The piston system is not that much heavier, the main weight difference is due to the DI system had an ultralight Faxon Bolt Carrier group. The realistic difference is the weight of the solid Op rod and marginally heavier gas block on the piston system. If the carriers were both the same weight, the difference would only be in the weight of bulkier piston gas block and op rod.
When it comes to weight, at the moment DI is the clear winner simply because a builder has the option to use a lightweight or ultra-light carrier with an adjustable gas block which can drop a huge amount of weight. If you are adjusting the gas pressure down and have a lighter carrier, you certainly do not need a heavier H2 or lardy H3 extra heavy buffer and can stick with a lighter standard carrier. All the way around the DI has the advantage to shed weight in a way that the piston systems do not.
TESTING - RECOIL & SHOOTING COMFORT
This was a rather interesting test because from an overall recoil impulse perspective, both felt very similar but the DI felt smoother and the piston system recoil... felt longer. The piston system always felt like something mechanical was happening all the way through the cycling process. I could literally feel the shot impulse, the op rod pushes back, the action cycling and the op rod and bolt resetting back into battery. The piston gun impulse feels like a process is going on vs just a recoil pop.
With that noted the recoil itself was similar between the two it just felt different. The DI system you identify the cycling process at 80% recoil and 20% carrier resetting into battery which overall felt lighter. The piston system was about 25% shot impulse, 25% op rod push back, 25% action cycling, and 25% the op rod and carrier final lock up. The piston system is different recoil feel that some will not like and some will. This was really a tie between the systems.
TESTING - ACCURACY
Strapping on a Burris 4.15-14 XTRII scope, the accuracy was so statistically similar, it is pointless to compare them. What I will note is that with plain old 40gr Fiocchi VMAX rounds I could consistently deliver ¾” 100-yard groups with either upper. These Faxon Match barrels, skinny or not, are super accurate.
TRICKS ONLY FOR THE DI GUNS
In addition to the un-natural weight reduction options, the caliber world is far more open to DI than the picky about pressure piston guns. Piston guns need the extra pressure to operate, but DI are much more flexible. Rounds like the 7.62x39 and 300 Blackout subsonic are still problematic for piston guns. Another trick the DI guns do is with the take-down kits like the Dolos system. The Dolos system allows the barrel to be removed from an upper still attached to a lower and as you can see with this kit stowed in small Hazard 4 Plan B sling pack, it delivers something exceptionally small and packable. Piston system are extremely sensitive to precise alignment and spacing of the op rod in relation to the bolt. A system like the Dolos cannot be used with piston guns due to the tolerances it requires for operation.
FINAL THOUGHTS
After 1000 total rounds, I wanted to shoot the piston system more than I did the DI system. The recoil was the same, but there was something about that mechanical almost steampunk feeling that made it fun to shoot. All along, I knew the piston upper would stay clean and keep shooting. From a reliability perspective, both systems ran perfectly which might have changed as round counts increased.
Would I rather have a piston gun in the zombie apocalypse? Yes. The biggest problem I have with the short stroke piston retrofit kits is that they literally have to be set up perfectly, exactly, precisely or they do not run right. Once setup and locked down, they run perfectly and amazing and worth the effort. Once they are set up, they seem to deliver a gun that can run what seems like forever. If you have not tried a Superlative Piston retrofit piston build, it is a worthy build.
ABOUT SUPERLATIVE ARMS
There are a lot of different gas blocks on the market, however Superlative Arms versions are absolutely the only ones I recommend to people. Over the years I have actually weeded out all the other gas blocks I have and converted over to Superlative simply because of several reasons. They are the only adjustable gas block which has not failed, reliably holds the setting, and has a unique venting feature which vents the excess gas instead of restricting it. This ultimately delivers a DI adjustable gas block which stays clean and does not carbon over like other gas blocks.
Superlative Arms has also developed one of the easiest and most reliable retrofit piston kits available on the market. They provide good instructions and a very premium tier quality product that I personally have run well over 10K rounds through one of the conversions I did. If you are thinking about a Piston system, you might want to just convert your existing rifle or use this system for a new build.