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  1. #1
    Bill Alexander

    Examination of piston accuracy (6.5 Grendel)

    I posted this quick review over at M4carbine but now the Grendel site is here this is more appropriate

    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.

    Bill Alexander

  2. #2
    Bill, would you please explain the dynamics behind the presumption of less accuracy from a piston operated rifle?

  3. #3
    I've never understood that either. It sounds like something that has to do with some theory, and was accepted as fact by the entire community without any real world testing to verify the claims.

    I can't tell you how many times I've had someone say that a piston design is inherently less accurate than a DI design AR 15 and that it is a waste of time.

  4. #4
    I vaguely remember reading a report (don’t remember where) a good while back by someone reporting accuracy degradation from a piston system. However, it was not a lot and it was a standard A-4 pattern rifle. Other than that one, I have never seen anything else from someone who actually tried it.

  5. #5
    I wonder what would happen if you went the other way and didn't drill a hole at all. What effect would making it a bolt action have on accuracy? I've always wondered how much accuracy if any is lost using semi-automatic actions.

  6. #6
    This probably came from the fact the AR is more accurate than the AK. I think that led to a urban legend that all piston systems were less accurate. My.2C

  7. #7
    Chieftain Variable's Avatar
    Join Date
    Mar 2011
    NRA/GOA/SAF Life.... West Virginia
    There was a lengthy post about the subject by another member (I forget who?) on the old forum. The underlying theory (as best I recall) had to do with reciprocating mass. A DI gun has all of the reciprocating mass (bolt, bolt carrier, buffer, action spring, etc) traveling in direct line with the bore axis, whereas piston guns have at least a significant portion of their reciprocating mass above the bore axis. This will cause additional off axis "upset" during the time the weapon is cycling. If you watch slow motion videos of some weapon systems you can actually see the torque and vibration that this causes. A slow motion of the AK looks like a train wreck on some of them. You can actually see lots of receiver flex and other stuff going on. How much this actually applies to the AR platform I have no idea, but I haven't seen any match guys running to the piston system for their match guns. It could be that the whole issue is overblown, but maybe not also. If Bill A. (or others) can turn out accurate piston guns, then people will probably ease up on this prejudice.

  8. #8
    Quote Originally Posted by Variable View Post
    How much this actually applies to the AR platform I have no idea, but I haven't seen any match guys running to the piston system for their match guns.

    The 3gunners definitely prefer less reciprocating motion. Lightened bolt carriers with adjustable gas systems so there’s just enough energy to cycle the system and nothing more. The only 3gunners using piston systems that I can think of is the FNH team who use SCARs.

  9. #9
    I'm pleased to see actual data about accuracy on piston vs direct impingement systems. I would also like to see data about the recoil impulse of similarly built rifles where the only difference is piston vs direct impingement.

    I would be interested in both the amount, and duration of the recoil impulse, and the perceived feeling of the recoil impulse.
    Realizing that various muzzle brakes affect the recoil impulse, I would also be interested to find out about how the three or four top brakes affect recoil impulse; in other words, how effective they truly are.

  10. #10
    This link was discussed on another forum:

    and is a comparison of the two methods keeping a lot equal. A dominate concept presented is impact of the piston on the bolt. However, I do not see much impact as the bolt and spring loaded piston appear to be in full contact before the trigger is pulled. The piston seems to push the bolt rearward when the gas gets behind it.

    I enjoy this cleaner, cooler action in my LWRC rifles (4) which are accurate for me (especially the .308 Repr), but I am am having a bit of issue with my #2 6.5 Saber equipped by factory with Adams piston. (10% stove piping discussed in Troubleshooting). The piston designs are different, and the Adams bolt spring is still a mystery to me. But when it shoots it is a tack driver at 200 yards! But so does my direct impinge #1, dirt and all.

    And my other piston rifles Scar 17 and FNAR have not missed a lick yet, nor have my Galil or ak-47. Now I have had a few issues with a pair of old M1 Garrands but cleaning the piston tube corrosion solved that.

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