I had what sounds like the same problem when I assembled my CMMG lower a few months back (I'm still waiting on my upper). Velocity wrote:I posted the below on as well - my one reply so far was that the RR lower appears out of spec. It just makes it a pain in the rear to assemble/disassemble/clean. I think what the above really means is: We need to check our tooling and stacked tolerances more often.Īn "overly tight" fit between the upper and lower on an AR does nothing for accuracy. You may have some difficulty separating the upper and lower, with barrel up and rifle unsupported (hanging in the air) grasp the rifle by handguard and give a "smack" to the top of the buttstock, you may need to do this repeatable, usually 5-10 times will do the trick. You may see some of the anodized removed from the rear lug at the corners, AGAIN, this is normal.Ĩ. This will seem scary to a beginner but it is natural for a tight fit.ħ. When they are seated (little to no gap between upper and lower) lightly tap the rear takedown pin into place. Open and close, repeat being careful to keep the halves aligned, do not slam them together but continue to use increasing force to fit together, when almost completely together use a nylon or soft rubber mallet to tap upper receiver into final position.Ħ. Slowly push the 1/2's together, they will stick (THIS IS NORMAL) it will most likely not close completely at first this is normal.ĥ. Ensure the rear takedown pin is completely out in the open position (detent is holding)Ĥ. Lubricate the rear lug completely with a light grease (best) or oil, lube the inside of the receiver also.ģ. Assemble the front pivot pin to the upper.Ģ. Works every time.Īll of the Rock River Arms lowers are an extremely tight fit, they need to be worked in the following way :įor the love of God make sure there is no rounds in rifle and it is on safe, the hammer will need to be back in the cocked position!!! Our lawyers make us tell you that death is possible if this is loaded when you perform this, we like our customers please do not shoot yourself!ġ. I use this method during assembly rather than removing metal from the lower receiver or the upper receiver lug. The following is from RB Precisions website. The same issue happens with those parts in place. In this picture, I've also removed the buffer/buffer spring, and the bolt carrier group, just to eliminate those as problems. Here's a picture of the CMMG upper and Rock River lower "about" to close - you can see the wear marks (circled) where the upper and lower rub when I try to pivot the upper into the lower. It simply won't close all the way - feels like the RR lower is too "short" by a millimeter or so to fit either of the uppers correctly. The front pivot pin goes in fine, but when trying to "pivot the upper closed" the rear takedown catch on the upper seems not to want to "mate" with the Rock River lower. That's bad, but what's worse (and the real reason for my post) is that neither my Bushmaster NOR my CMMG upper will match up with the now-assembled Rock River lower. ![]() Everything went together ok, with the exception of the rear takedown pin being VERY tight - was not hard at all to get in into the lower, but once the rear takedown detent and spring were in place and the stock was tightened, it's VERY tough to push out the takedown pin. ![]() I bought a Rock River stripped lower (marked LAR-15 / 5.56cal) and a DPMS lower parts kit with a Tapco stock - took until today to get it all assembled. Just before the election, I decided I wanted to get another lower to match with the complete CMMG upper, and put my original Bushmaster back together. Life is good for a year with my Bushy lower and CMMG upper - no problems. I wanted a flattop, so I bought a CMMG complete upper and did a switch with the original permanent-carry-handle (A2?) upper. My first AR15 with a Bushmaster XM15-E2S. I posted the below on as well - my one reply so far was that the RR lower appears out of spec.
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