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Posted: 9/30/2010 12:09:13 PM EDT
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I was recently at the range with a couple co-workers, one of whom was shooting a relatively new CMMG piston gun. We had a jar of Tannerite that we set 50 yards distant and then set about trying to detenate said jar. The gun was wearing a 1-4x optic that had been zeroed (but not grouped) at 100 yards immediately prior to shooting at the Tannerite. Long story short, approximately 50 rounds of ammo was fired between 3 different shooters from a sturdy bench, rifle rested snugly atop a sandbag and the target remained intact. The rounds were impacting all around the target, creating a fairly noticeable beaten zone, but the target itself was unscathed save for a glancing blow to the screw-on lid. We ended up blowing it up on the first shot from a Colt 6940.
The gun itself was equipped with a 1/7" twist 16" M4 contour barrel, and the ammo used was Silver Bear 55 gr. We surmised that the piston operation had a detrimental impact on the rifle's accuracy due to the barrel's light contour, and that perhaps a heavier barrel would not have been so affected. I realize that the ammo we used is not known for stellar accuracy, but seriously, 50 yards and 50 rounds between 3 fairly experienced shooters with zero hits on target?? Has anyone else had any similar experiences with piston operated rifles? |
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Quoted: I was recently at the range with a couple co-workers, one of whom was shooting a relatively new CMMG piston gun. We had a jar of Tannerite that we set 50 yards distant and then set about trying to detenate said jar. The gun was wearing a 1-4x optic that had been zeroed (but not grouped) at 100 yards immediately prior to shooting at the Tannerite. Long story short, approximately 50 rounds of ammo was fired between 3 different shooters from a sturdy bench, rifle rested snugly atop a sandbag and the target remained intact. The rounds were impacting all around the target, creating a fairly noticeable beaten zone, but the target itself was unscathed save for a glancing blow to the screw-on lid. We ended up blowing it up on the first shot from a Colt 6940. The gun itself was equipped with a 1/7" twist 16" M4 contour barrel, and the ammo used was Silver Bear 55 gr. We surmised that the piston operation had a detrimental impact on the rifle's accuracy due to the barrel's light contour, and that perhaps a heavier barrel would not have been so affected. I realize that the ammo we used is not known for stellar accuracy, but seriously, 50 yards and 50 rounds between 3 fairly experienced shooters with zero hits on target?? Has anyone else had any similar experiences with piston operated rifles? Try different ammo and check the optics mounts. What you are describing is not a piston issue. |
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Before installing the ARES GSR-35 piston system on my 16" heavy match barrel carbine, I performed some informal accuracy testing from a distance of 50 yards using the direct impingement gas sytem. The forearm of the rifle was placed on a sandbag resting on a wobbly table. There was no support for the butt-stock (nor for my elbows.) The ammunition used was one of my hand-loads, using Sierra 52 grain MatchKings and VihtaVuori N135 powder. As well as being an extremely accurate load, this load has functioned flawlessly in every AR-15 I own.
I fired three 10-shot groups in a row that measured, 0.558”, 0.408”, and .570” for an average group size of 0.512”. The targets are pictured below. 
before conversion 
after conversion 
I began function testing of the newly converted carbine by loading and then firing a single round at a time from a magazine for the first ten rounds and then 3 rounds at a time for the next several magazines. I experienced multiple failures of the bolt to lock back after the last round of the magazine was fired. I don’t know if this was do to the slight binding of the mechanism I mentioned above, or if the system just needs a little “breaking in.” After approximately 30 rounds the bolt began to lock back consistently. There were no further malfunctions during testing. I performed informal accuracy testing with the GSR-35 system installed on the carbine in the same manner as described above. The three 10-shot groups that I fired from 50 yards measured, 0.677”, 0.501” and 0.879” for an average of 0.685”. (targets pictured below) This does demonstrate a decrease in accuracy (larger average group size) using the gas piston system compared to the direct impingement system, albeit a rather small one at approximately 0.35 minutes of angle with this 16” HBAR. 
Also, while there may be a slight decrease in accuracy when using the Ares system, the accuracy is still more than adequate for my intended use. In the last test session I fired the 10-shot group pictured below from 50 yards in an informal accuracy check, using handloaded 55 grain V-MAX bullets. The group measures 0.632”. While not minute of angle, it is certainly good enough for government work. 
Using the Ares upper and 55 grain FMJ handloads, I fired a quick 10-shot group from 50 yards on a 300 yard E2 silhouette target reduced for 50 yards. The results are pictured below, showing that while the Ares system might not produce MOA accuracy, it is certainly good enough for practical shooting situations even when using FMJ rounds. 
100 Yard Accuracy Test Formal accuracy testing of the Ares carbine upper was done following my usual protocol. Three 10-shot groups were fired from a distance of 100 yards off a concrete bench using front and rear bags to support the rifle. A Leupold VARI-X III was used for sighting. The ammunition used was one of my standard handloads using Sierra’s 52 grain MatchKing. Wind conditions were monitored using a “Wind Probe” (and it’s a good thing because it was quite windy during testing.) I fired three 10-shot groups in a row from the Ares carbine with the extreme spread of the groups measuring: 1.04” 0.99” 1.20” The average extreme spread for these three groups is 1.08”. This is right on par with what my direct impingement 16” Colt HBARs will do. 
The three 10-shot groups from the Ares upper were overlayed on each other using RSI Shooting Lab to form a 30-round composite group. The mean radius for this composite group was 0.32”. The composite mean radius of three 10-shot groups fired from my 16” Colt HBAR was also 0.32”! Shown below is a comparison of the accuracy of the Ares upper with three different 16” Colt barrels. 
Ares GSR-35 on Colt 6922 
The first thing I noticed when installing the Ares unit on this barrel is that the gas spigot binds on a portion of the barrel near the front sight base. This did not occur with the heavy barrel that I used in the original test. One has to wonder what effect this might have on accuracy. Arrow indicating area where gas spigot binds with barrel. 
Original HBAR setup without any barrel binding. 
I wanted to use a free float hand-guard with this set-up to avoid the problem of the unit binding on the Ares M4 hand-guards. Since Ares still has yet to release their “upside-down KAC” free-float rail designed for this unit, that was not an available option. The only commercially available free-float hand-guard that I’m aware that will work with the Ares unit without requiring an alteration of the hand-guard is the A.R.M.S. SIR system. I used the #50 slim-line, bi-level military version SIR. The highest portion of the Ares unit clears the underside of the SIR rail by approximately 0.030”. A.R.M.S. SIR with Ares GSR-35. 
100 Yard Accuracy Comparison Prior to installing the Ares GSR-35 unit on the 16” M4 barreled upper with the ARMS SIR free-float rail, I tested the accuracy of that upper in its direct impingement configuration from a bench-rest at 100 yards. Using hand-loaded 52 grain Sierra MatchKings, three 10-shot groups were obtained which had extreme spreads of: 1.59”, 1.55” and 1.73” for a 10-shot group average extreme spread of 1.62". While not the most accurate barrel I own, it’s still perfectly suitable for “government work.” The three 10-shot groups were over-layed on each other using the RSI Shooting Lab software program to obtain a 30-round composite group. The mean radius of the composite group was 0.54”. After the upper was converted to the GSR-35 piston system, accuracy testing continued in the same manner as above. Three 10-shot groups were obtained using the same load of 52 grain Sierra MatchKings. Those groups had extreme spreads of: 1.77”, 2.54” and 1.85” for an average 10-shot group extreme spread of 2.05". The mean radius of the 30-round composite group created from those three groups was 0.69”. This demonstrates a decrease in accuracy of approximately 0.45 MOA for the ARES system. 
On the outside chance that the increase in group size was caused by the Ares configuration simply “not liking” the 52 grain MatckKing load, I fired an additional three 10-shot groups using a hand-load of 55 grain V-MAX bullets charged with a different powder. Those groups had extreme spreads of 2.65”, 2.14” and 1.78” for an average 10-shot group extreme spread of 2.19" with a 30-round composite mean radius of 0.72”. As I mentioned earlier on this page, one has to wonder if the Ares unit binding against the barrel is having a negative effect on accuracy.
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I have a new LWRC M6A2 16" 5.56. I started my first accuracy shooting today after around 100 rounds of break-in.
At 100 yards, front end supported; 65 degrees in very light rain, Outside of one particular load, I am averaging .75" 3-shot groups across almost all ammo. Specifics: PMC M855 (62gn FMJ AP) spread= 2.50" (this was two seperate groups for verification) Federal M855 (2010 production) Spread=0.687" Federal M193 (2009 production) Spread=0.625" Handload 69gn Speer HPBT - Spread=.813" I'm impressed with most of it except for the PMC which was fairly cheap...obviously the LWRC doesn't like it. I'm also surprised that the heavier bullets didn't outperform the 55 gns. This is better than any DI gun I've owned, and I've owned some nice ones, so I would be very hard pressed to make any blanket statements about piston guns having an apparent lack of accuracy compared to DI. |
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Just returned from the range with my Osprey build, really no significant decrease in accuracy since the conversion. Bear in mind that this is a SHTF carbine, and my goal is 1.5 MOA at 50 yards, 2-2.5 MOA at 100. Combat accuracy vice target accuracy. It achieved this as a DI and maintains it as a Piston. The thing that still impresses me is how clean the BCG is after running a few hundred rounds. |
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Quoted:
I have a new LWRC M6A2 16" 5.56. I started my first accuracy shooting today after around 100 rounds of break-in. At 100 yards, front end supported; 65 degrees in very light rain, Outside of one particular load, I am averaging .75" 3-shot groups across almost all ammo. Specifics: PMC M855 (62gn FMJ AP) spread= 2.50" (this was two seperate groups for verification) Federal M855 (2010 production) Spread=0.687" Federal M193 (2009 production) Spread=0.625" Handload 69gn Speer HPBT - Spread=.813" I'm impressed with most of it except for the PMC which was fairly cheap...obviously the LWRC doesn't like it. I'm also surprised that the heavier bullets didn't outperform the 55 gns. This is better than any DI gun I've owned, and I've owned some nice ones, so I would be very hard pressed to make any blanket statements about piston guns having an apparent lack of accuracy compared to DI. Good info, thanks for posting. |
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something I have always said. you start with a crappy barrel add a piston system and it still shoots crappy, people blame the piston system. back to the OP: a CMMG barrel and a Colt in my humble opinion, are not in the same class, so you can not say it was the piston system. how do you know it was not the combination of crappy ammo and a not so great barrel?
The poster using the Colt barrels did a better test, starting with a better barrel. any difference the piston system makes, is not going to be significant for the majority of the people out the.....if some of them add a couple tenths to our groups, is that going to make you miss the kill shot on a dear or other target? I think the adrenaline rush from a "shoot for your food/life" situation will have more effect on your accuracy then the piston system will. |
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Quoted:
if some of them add a couple tenths to our groups, is that going to make you miss the kill shot on a dear or other target? I think the adrenaline rush from a "shoot for your food/life" situation will have more effect on your accuracy then the piston system will. IMO in combat stress situations, where you have the adrenaline rush, moving, partially obstucted/in profile target making the shot harder wouldn't you want as little problems from the rifle as possible? I don't have personal experience with this but it would seem to me there is a HUGE difference between a paper target faceing you head on and holding still vs. a live, running ducking, laterally moving target. One assumption people make for example is that they are aiming dead center when they shoot, so a couple MOA woun't matter. YMMV |
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