Posted: 9/9/2011 5:35:27 AM EDT
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Hello, I bought a dedicated CMMG 9mm lower through Brownells, and dropped on a new RRA 9mm complete upper from ADCO. Everything assembled easily (ejector and BHO came already installed).
Here's the problem: When I assemble the upper to the lower with the bolt closed, the bolt won't pull back. I have to open the receivers to pull the bolt back. If I remove the rear takedown pin and allow the receivers to separate just slightly, and then pull the bolt back into the receiver extension, reinsert the rear takedown pin, and ease the bolt forward, it stops about 1/4" from the chamber, where it again can't move. I'm not sure of the problem––perhaps the ejector is being pressed against the bottom of the bolt? Is there a troubleshooting checklist I can run through to figure out why this lower isn't compatible with the RRA upper/bolt? Thanks! |
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Quoted: Thanks for the very fast reply. I had the bolt ramped by Adco before it was sent out. Is their version of ramping incompatible? Assuming so, would replacing or removing the bolt catch solve the issue? Thanks again. Their ramping my not produce enough clearance. A RRA 9mm bolt catch should the trick. |
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For anyone considering this combination –– CMMG dedicated lower with RRA upper ramped by ADCO, I can confirm that CMMG's bolt catch and ADCO's ramping are not compatible. The RRA bolt catch made the combination work. The CMMG bolt catch sticks up higher, into space cleared out by bolt ramping. ADCO mentions in their shop work description that their ramping is a compromise, i.e. not as deep as some others.
This is not a criticism of either company, and I sincerely appreciate CMMG's rapid reply to my issue. I'm just noting the incompatibility, to try to save future purchasers some money and grief. Pic comparing CMMG bolt catch to RRA bolt catch. Note that the CMMG bolt catch is bigger (I'd assume thereby less likely to break), but it requires a deeper bolt ramp than ADCO performs. FYI only. |
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I have both a CMMG 9mm lower....love it.....and a RRA lower with a mag well adapter. The RRA upper drug on the RRA lower at the bolt catch. It has wear marks on it! I switched the RRA upper to a Cav Arms lower with RRA mag well insert and now all is well. For my dedicated CMMG lower I picked up a CMMG bolt...perfection! No drama from this and all is well.
My point is simple, watch what parts you mix and match when building a 9mm AR from scratch. My CMMG setup...using all CMMG parts....works great. The RRA parts were not as simple to get running. Oddly with the RRA upper a Cav Arms lower worked out better than using an RRA lower and that seems odd but its the way it was. It seems that little bolt catch thing is a bigger deal than what you would expect. |