Got all the parts together today. Using a CMMG dedicated 9 lower. An Anderson lower parts kit and the upper was a complete upper found on GB. Every thing went together great except when it came time to put the upper on the lower and go test fire. The lower take down pin hole would not line up to get the pin to go through to close the two assemblies. trying it a different way by putting in the rear pin first and then the front pivot pin the two halves will line up and the front pin can be pushed into place but only after the bcg is locked to the rear. When you hit the bho to close the bolt it moves forward 3/4 into battery and comes to a halt. I then have to separate the two halves because the bcg becomes stuck in that "almost closed position" and will not move forward or back using the charging handle. Any suggestions would be great.


After checking individual parts like the ejector and bolt catch, it was found that the bolt catch was preventing the bolt carrier to move freely under spring tension. There was that much drag on the bottom of the carrier, so  a couple of minutes with a file to remove some material from the top of the bolt catch and now the two halves fit nicely together and it runs without and problems. Thanks to all that offered solutions.

Sounds like the upper or lower is off. I had a Colt slick side that I had to ream the rear hole on. It was off on every lower ( 7 brands ) that I tried it on. Make sure it fits everywhere else before trying this.

One more thing.  Make sure the mag adapter is out before doing any "work". And if another upper and lower is available for a swap, see if it will assemble.

Dedicated magwell, sorry. However, some 9 uppers are relieved to fit a mag and ejector.
They have a wider bolt raceway opening.




Edit, I remembered the upper relief.