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Discussion Starter · #1 ·
My friend has been wanting an AR for a while now. After a lot of research, he decided on a 300 black out build. He already had a S&W lower. He went with a CMMG complete upper, 16 inch barrel, olw pro gas block, and rifle length YHM rail. Complete upper was purchased through Model 1 Sales, but is CMMG. Lower parts were RRA 2 stage trigger group, and all of the other lower parts were from another name brand kit (can't remember which.) He plans on turning it into an SBR, and putting a suppresor on it. He has both his tax stamp forms submitted.

First of all was the wait. I thought he was going to get his tax stamps before the upper came in. He waited 6 weeks! When it arrived, we already had the lower built. Pinned on the upper, and we headed to the range. Short version of a long story is, my friend had waited 6 weeks, paid more than what he should have, and now has a nice looking SINGLE SHOT AR!

The rifle is short cycling. We tried all of the easy fixes; checked gas key, gas rings, buffer assembly, magazine, bolt carrier assembly, lubrication, cleaned chamber, etc. I had extra parts on hand, and we changed alot of parts out trying to find an easy fix. I've narrowed it down to the gas block. It's a low pro with set screws. I hate those, and they are notorious for misalignment. I actually tried to get him to go with a standard gas block/front sight, but everyone knows those are not "cool".

Fit and finsh looks good on all of the parts, but I'm disappointed in CMMG and Model 1 Sales. Both companies claim the uppers are test fired. Maybe they only fire one round during the test fire process. I think one of the companies dropped the ball, but I can't say which. In the end, it should be an easy fix, and he should have the rifle running in a couple of days. I know he could send the upper back, and either company would make it right, but I feel it can be fixed locally. I just hated watching his ear to ear grin fade away after each malfunction.
 

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Did Model 1 assemble the upper from CMMG parts? That would be odd.
CMMG does test fire the uppers with 10 rounds if I recall.

I have a 9" CMMG 300 BLK. I opened the gas port up. It's a pistol length gas system.

I know one of the engineers that worked with the 300 BLK for CMMG and opening the gas port was what he advised for optimal operation.
 

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Discussion Starter · #3 ·
I don't believe Model 1 Sales assembled the upper. It was a complete upper from CMMG. My friend just ordered it through Model 1 Sales. Everything I have read points to the 16 inch barrels being the hardest to keep running. I should also note my friend was shooting subsonic loads. He has since fired super sonic loads, and the rifle cycled. It still did not eject consistent, nor did it lock the bolt back when empty. I'm with you Shadow1, I think the gas port needs to be opened.
 

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I have a 300 blackout that was built locally. Haven't fired it yet, but I am this weekend. I will keep the gas port in mind if I have any problems with mine.
 
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