Examination of piston accuracy (6.5 Grendel)

This may not be the correct section but as the basis for the experiment is a 6.5 Grendel it seems appropriate to post here.

During the past few months I have been slowly examining the effects of using a piston type operating mechanism on the accuracy of 20" DMR type build. The baseline rifle for the study is our existing GDMR which houses a 20" Satern barrel with rifle length operating system in a monolythic upper, in which the railed handguard forms an integral part of the whole reciever. The gas system uses a conventional gas tube with a non adjustable gas block and a port size of 0.093" which is nicely balanced for most loads. The barrel profile is medium heavy at 0.980" dia under the handguards with 8 off 3/16" flutes. Typical fleet data from building and testing these rifles during production shows the accuracy to be around 0.5 inches at 100 yards for a 10 round group, testiment to the Satern barrel.

The corresponding piston operated rifle came into being due to the duty cycle of one of the customers. Extensive use of suppression accelerates fouling and heat transfer so a piston design was requested. The rifle shares a common chassis and gas system length, although the barrel fluting is heavier to help with weight. Gas port diameter increases to 0.106" to balance the Adams Arms gas piston used in this build with most ammunition.

The testing was started with the preconception that the gas piston would open the group sizes. It was therefore surprising to see that accuracy was identical to that seen in the direct impingment units. Recoil although subjective felt somewhat smoother and more straight line. Responding to the possibility that the shift in accuracy was being masked by the barrel quality, an identical unit was made using a Begara button rifled blank. Again the rifle shot exceptionally well and it was clear that the piston mechanism, although it imposes off axis loadings has little effect on the accuracy of this rifle layout. To verify accuracy was examined out to 600 yards.

It is hypothesised that for the 6.5 grendel, the piston type design, when restrained within a suitably ridgid reciever has no negative effects on the accuracy of the host weapon. Timing is still set by the port position and unlock timing is adjusted by the same.

This is a very rough summary of work to date and more comprehensive testing is required. Additionally it is proposed to build a series of shorter barrel weapons to se how these respond.

Questions, suggestions and observations please.

Billl Alexander

Interesting writeup:)

Even a piston cannot screw up a system derived from a true monolithic chassis, and a premium barrel chambered in an inherently accurate caliber.

I have always been sceptical of the piston for a truely accurate rifle as it imposes not only off axis force but also adds movable mass to the barrel neither of which is conducive to ultimate accuracy. The testing we have, has begun to dispell this idea but questions such as accuracy over the life of the unit still remain.

I am extremely pleased with these piston DMR set ups I will build an additional two this week to see how they compare. The Begara barrels may not be the best test subjects as they also are accurate but it is hoped that they will not mask problems like a cut rifled barrel will.

Bill Alexander

What is the typical shift in accuracy from a DI to Piston for say a 5.56?

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Originally Posted by TXBob

What is the typical shift in accuracy from a DI to Piston for say a 5.56?

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I do not notice any difference in accuracy or recoil when switching between DI and piston on my MRP. That being said, I'm not a bench shooter.

I seem to recall some high-speed videos of an LWRCI rifle firing without the top rail. It appears that the bullet clears the barrel before the piston begins to move. This would suggest that there is little if any ill effect on accuracy from the piston.

FYI, I'm not a piston guru and piston ARs are a minority of my stable. I just have a couple for suppressed SBR work.

Very interesting stuff! Thank you!

Thank you for sharing. Accuracy is one of the main liabilities I read about in a piston platform. Per Trident's comment, I do not see myself purchasing a non-monolithic upper in the future, however I am still open to a piston.