Short stroking with CMMG 16 inch upper - conversation/CMMG

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Downrange
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Short stroking with CMMG 16 inch upper - conversation/CMMG

Post by Downrange » Wed Jul 25, 2012 12:14 pm

I just hung up the phone with Brent at CMMG about my new 16 inch M4-style upper FTF issues with Rem 220 gr. subsonic.
His bottom line recommendation is to change the buffer to an H3 or tungsten model that is heavier, to hold the bolt in battery a bit longer and increase the gas action that way. He also said the action will loosen some as the rifle is fired more, and that would help. He also mentioned the Rem 220 gr. was a bit underpowered and recommended using the PNW 220 gr subsonic.
There was another post in this section that mentioned an "adjustment" to the gas port, but he said the M4 style gas port I have is not subject to adjustment.
My question is does this all make sense to you guys? Apparently the buffer is about 30 bucks, so it's not a huge expense, if it fixes the problem. Is it true the Rem 220 is underpowered? I find it hard to understand it would be so, and, especially as I'm running a can, I expected no problems in running the subsonic ammo. I have not chrono'ed anything yet, but that's on my short list to see what's up.
Any ideas? Should I just get the H3 buffer?
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mdawson
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Re: Short stroking with CMMG 16 inch upper - conversation/CM

Post by mdawson » Wed Jul 25, 2012 12:28 pm

Sounds like some good advice. I wouldn't worry about the gas port.

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Re: Short stroking with CMMG 16 inch upper - conversation/CM

Post by marsh-vol » Wed Jul 25, 2012 1:50 pm

The other thread had an upper that was self-assembled. He didn't get the gas block properly aligned, so the Samson rep "adjusted" it to proper alignment.

For your rifle, I'd try a box or two of supersonics to see how they function. It may be a break in issue and supersonic loads should speed up the break in, but it will help narrow the problem.

The Remington subsonics are loaded to be subsonic across a broad range of guns and locations. You may be able to load your own for your location a little hotter.

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L1A1Rocker
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Re: Short stroking with CMMG 16 inch upper - conversation/CM

Post by L1A1Rocker » Wed Jul 25, 2012 1:54 pm

This has happened before with CMMG uppers. The gas block is not adjustable but the position of the gas block relative the the gas port may need to be adjusted. If the gas opening on the gas block is not correctly alligned with the gas port from the barrel then you essently have the gas "pinched" off. After doing some shooting you can take the gas block off and look at the soot ring around the gas port of the gas block. If it is half moon, or cresent moon shaped it is not alligned correctly. Adjust its position on the barrel accordingly.

Also, someone in the forum had a problem with the gas key not being correctly staked and it was loose. You may also want to check there.

Good luck.

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Downrange
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Re: Short stroking with CMMG 16 inch upper - conversation/CM

Post by Downrange » Wed Jul 25, 2012 2:12 pm

Thanks for all the replies. I will probably try the heavy buffer first, as that seems to make sense and isn't too expensive. I will report back here after more testing.
I have shot a few rounds of full power, and they cycle fine.
The upper is accurate - no problem there.
The world is made up of two kinds of people: the kind who believe that everything not specifically prohibited is legal, and those who are always looking for more things to prohibit.
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