AR build help!!! - Page 1

AR-15 » Troubleshooting

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Posted: 11/12/2018 9:08:28 AM EDT

I put together an AR for a buddy yesterday, I’m not new to building, but I have an issue I just can’t figure out. The lower is a stag arms and he got an M&P lower parts kit. My buddy assembled his upper (he owns a company that builds AR’s) the BCG is an Expo arms the barrel is a bear creek arsenal. When the lower is joined to the upper the pivot pin is so tight it won’t go in by hand (for starters) once joined the bolt will only come back 3/4”. I put his upper on my lower, no issues. Put his lower on my upper, no issues. I’m at a loss, any ideas???

Posted: 11/12/2018 9:15:06 AM EDT

[#1]

So it's only interfering with the stage lower on his upper, did I read that correctly?

Posted: 11/12/2018 9:27:47 AM EDT

[#2]

It seems to work fine on a cmmg

Posted: 11/12/2018 9:29:35 AM EDT

[#3]

I can’t see where it’s hitting to determine the problem

Posted: 11/12/2018 9:33:05 AM EDT

[#4]

All I know to advise you to do is take a few high quality receivers and a caliper and start measuring... it would be nice if you/we had a blueprint of correct specs clearances and tolerances...TDP...

First thing I would want to check is the pin hole location.

Posted: 11/12/2018 9:42:33 AM EDT

[#5]

Is the bolt unlocking from the extension when hand cycling with the charging handle? Are you saying that the carrier gets stuck after moving rearward 3/4'' when using the charging handle?

Posted: 11/12/2018 11:00:12 AM EDT

[#6]

Sounds like a tolerance stacking issue, your buddy's lower and upper are on the high end of the tolerance scale and do not fit well together. You can try taking measurements to find where the issue is, but in the long run it may be easier just to use a different upper receiver.

Posted: 11/12/2018 1:22:19 PM EDT

[#7]

I haven’t haven’t tried it with the bolt out of the carrier. That’s next on my list

Posted: 12/9/2018 12:12:59 PM EDT

[#8]

Did you ever figure out the problem?

Posted: 1/2/2019 2:15:41 PM EDT

[#9]

Back the buffer tube out one turn.

[ARCHIVED THREAD] - AR build help!!!

AR-15 » Troubleshooting

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