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russ 01-16-2010 08:51 PM

Piston power
So I just ordered my CMMG piston upper. I plan on building an M4 knock off with it on my current CMMG lower. Anyone have advice on piston driven AR's to impart to a noob? I plan on swapping the plastic handguard for a quad rail of some kind, but I need to get it in my hands and see what will fit first. I really dig the look of the Vltor CASV rail system, but I'm not sure it will clear the piston setup. Not to mention it's not cheap.

slowryde45 01-16-2010 10:23 PM

I'm not sure I understand the question. You say you have ordered a CMMG PISTON upper, is that a Complete Upper assembly or parts? So if its a complete upper, all you have to do is put it on and push in the two pins.

What advice are you seeking?

Gatekeeper 01-16-2010 10:51 PM

Think he wants to know if standard aftermarket quadrails and handguards will fit on a piston driven upper or if special handguards are needed to clear the piston assembly.

slowryde45 01-16-2010 11:07 PM


Originally Posted by Gatekeeper (Post 213897)
Think he wants to know if standard aftermarket quadrails and handguards will fit on a piston driven upper or if special handguards are needed to clear the piston assembly.

Ahhh...I see. Well, first you'd have to wait until you get that upper in hand to see how much room you have. Oh wait, you said that. could call in to CMMG, and ask their Gurus what will and won't fit. That will at least get you in the ball park.

And yes, Vltor does cost. Most of the upper brands will. Know that even brands like Yankee Hill or Midwest will run you around $140 and up for free float rails. As you go up the ladder, Daniel Defense, Troy, Vltor, etc, will start quite a bit more. So you could say "Nice = PRICE" :p

russ 01-16-2010 11:17 PM

I have ordered it and expect it here by the end of next week. I guess on the advice thing I was looking for maintenance or issues unique to piston setups. I've read about the carrier tilt issue and it seems that some carriers have the issue while others don't.

richardnorton 01-26-2010 03:07 AM

piston upper
Russ always go with a piston system if available, bolt and bolt carrier run cooler and cleaner as the piston keeps the burnt powder and hot gasses out of the bolt carrier. They just work more often than the gas inpungment systems of the standard AR-15/M-16 and no small tube to get clogged with unburnt powder to cause a stoppage. After more that 30 years in the Army I learned to hate the M-16, so I shoot the SIG 556, and the new ruger SR556 looks good and now everyone is making a piston upper. After you put the upper on the lower you will have to see what will fit.

russ 01-31-2010 06:33 AM


Originally Posted by Class273 (Post 221825)
Could a guy put a CMMG piston upper on a "your brand here" lower and go shooting post haste?

As to the OP, I think if you look around at different rail manufactures they will often note if their setups are piston system compatible.

You can absolutely put the piston upper on any mil-spec lower and go shoot.

As far as rails go, what I want is a longer rail that will extend out past the front gas block, but I'll have to wait until I get it in my grubby little hands to see how big the cut out needs to be to clear the gas block. My local guy is having trouble bringing one in, so I'm still waiting impatiently.

russ 01-31-2010 03:44 PM


Originally Posted by Class273 (Post 221837)
I have been told that if a bolt or upper has several thousand rounds thru it and you add in a new upper or a new bolt there can be issues. In essesnce, one is broken in while the other isn't relative to one another. Thoughts?

I can see where you might have issues putting a new BCG in a well worn upper and vice versa, but that shouldn't be a problem since the piston upper is complete with bolt, carrier, barrel, and upper receiver. And since we're talking about adding the complete upper to an older lower, I wouldn't anticipate any problems.

JonM 01-31-2010 09:41 PM

you can pretty much use any upper on any lower exepmpting things like .308 caliber AR's. my SR556 Panther M4 and Colt SP1 and olympic arms .45ACP upper all swap out and around just fine altho i have to use special offset pins to join the SP1 to milspec uppers.

the only problem i have witht he SR556 is that my CMMG .22lr conversion does NOT work with the ruger upper.

russ 02-02-2010 06:43 AM

No big deal.

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