CMMG 7.5" Piston Upper

[shadow65]

Took my new 7.5" CMMG Piston Upper to the range today. Never shot a piston AR so I wasn't sure what to expect.
The first thing I noticed was how smooth the recoil felt. I also noticed it didn't seem to have as much muzzle jump.
Once I got the sights dialed in at 25 yards, I moved to the 50 yard range. I was using a DPMS A2 rear sight I had set up for RIBZ. I set it to the 50 yard click and was shooting about 6" low. I thought that was really strange. It worked on my other DI 7.5".
I adjusted the sights to get the 50 right then went back to the 25. Set the rear for the 25 click and again was way off. This time high. WTF?
Instead of changing the front sight again I kept bumping the rear up. At the 50 yard setting, it was dead on at 25. Really odd but it works for me.
I'll get a BUIS and just dial it for 50 yards and bedone with it. It's a short range carbine anyhow.
Accuracy was good. Even shooting Wolf, I was about 2" at 50 yards with open sights.

I was also running a .308 suppressor on it. The can toned it down to about an unsuppressed .22. I can live with that.
With the .308 can, I have a suppressed 5.56 upper that is about 1/2" shoter than a 16" carbine.http://www.ar15.com/images/smilies/icon_smile_big.gif

I ran about 200 rounds through suppressed and it ran great.
I pulled the can off and started having short stroke issues. I had been running a 9mm buffer. I pulled it and put an H2 in. It ran better but still short stroked about 2 out of 5 rounds.
I'm sure the underpowered Wolf was causing this issue. It was close and would pick up the next round but the bolt would not lock back on the last shot.
It was also 25 degrees out.
I didn't have any full powered ammo with me but I have little doubt that it would cycle fine.
A carbine buffer would be another option to shoot Wolf.
I'm going to clean it good and lube the hell out of it. 250 rounds isn't even broke in.
I'm very happy with this set up. Suprisingly, when I got home it wasn't dirty at all, even running suppressed.
I look forward to shooting this one again real soon.
Dave
http://www.hunt101.com/data/500/medium/CMMG_7_5_004.JPG
http://www.hunt101.com/data/500/medium/CMMG_7_5_001.JPG

[AKTexas]

Very nice set up.

[abpt1]

Yeah try some factory M855 or some 75 hornady's its has a bit more oomph to it .

:thumbspbig:

[shadow65]

I just generally use Wolf to get on paper, then sight it in for my carry ammo.
But I do make sure all my rifles will cycle Wolf. Never know when that might be all you can get.

[Schuetzenman]

No doubt the Wolf was the issue. That stuff is weak, especially the 55 gr. stuff. Nice looking weapon. I've got a 16" middy piston rifle. You're right a piston rifle has a much smoother recoil stroke and the inside of the upper does stay clean. All that crap goes out under the handguards instead of in the upper and on the bolt group.

[shadow65]

16" length, suppressor included
http://www.hunt101.com/data/500/medium/7_5_Piston_003.JPG

[shadow65]

Went to the range again after a good cleaning. Still short strokes using Wolf and XM19. I'm sending it back for a look over. I will not own a rifle that won't cycle Wolf and I sure won't have one that only runs suppressed.

[308]

I'm thinking perhaps the gas port might need to be opened up a bit. I had problems a long time ago with my shorty FAL and opening the gas port up was the fix.

[shadow65]

Turns out there are two rings on the Piston plug. One had a chunk out of it and it got into the tube creating a burr. The piston was dragging on the burr and the piston could possibly been loosing pressure.
CMMG is replacing the piston block.
They tell me they are coming out with a redesigned plug.