I'm helping a friend build an AR.  He's bought a lower, a buttstock, and an LPK.  Had no problem assembling these, and the lower passes a function check.

However, I tried to put my Bushmaster upper on his RRA lower, and it wouldn't fit.  When the front takedown pin was in, the rear takedown lug of the upper simply wouldn't fit into the RRA lower.

When I compared my Bushmaster lower to his RRA upper, they were identical except for the inside rear of the lowers.  On the Bushmaster, the rear part of the lower actually flared out a bit before it curved and closed, whereas his RRA ended with a simple semi-circle at the end of the receiver.  It appears to me that in order to make the Bushmaster upper work with the RRA lower, we'd have to machine either the upper or the lower to get them to fit.  I had been under the impression that the DPMS/Bushmaster/RRA lowers were all identical.  Did I get that wrong?

There are variations in how the different manufacturers machine the FCG areas.  Rock River lowers are known to be rather tight in the rear takedown pin area.  I like their uppers but this is the reason I've kind of shied away from their lowers.  Good quality but I don't want to file or grind anything to make it fit.  

Try running it by Rock River on their page on the Industry Forum.  They may have some suggestions on how best to proceed.

There is no need to file or grind on anything when fitting an upper to a  RRA lower. Simply take a rubber mallet and lightly rap several times on the top rear of the upper and it will fit together.  You will have to rap on either the top of the barrel or the stock with the heel of your hand the first time seperating the two. After opening and closing it a few times it will begin to loosen up. Trust me after a few hundred rounds you will appreciate the fact that it fits together the way it does.

Quoted: There is no need to file or grind on anything when fitting an upper to a  RRA lower. Simply take a rubber mallet and lightly rap several times on the top rear of the upper and it will fit together.  You will have to rap on either the top of the barrel or the stock with the heel of your hand the first time seperating the two. After opening and closing it a few times it will begin to loosen up. Trust me after a few hundred rounds you will appreciate the fact that it fits together the way it does.

I'm not convinced I could make this fit with a few whacks of a hammer.  The difference in sizes is big enough that I'd actually be worried about damaging the lower.

Quoted: Quoted: There is no need to file or grind on anything when fitting an upper to a  RRA lower. Simply take a rubber mallet and lightly rap several times on the top rear of the upper and it will fit together.  You will have to rap on either the top of the barrel or the stock with the heel of your hand the first time seperating the two. After opening and closing it a few times it will begin to loosen up. Trust me after a few hundred rounds you will appreciate the fact that it fits together the way it does. I'm not convinced I could make this fit with a few whacks of a hammer.  The difference in sizes is big enough that I'd actually be worried about damaging the lower.

I watched Robert at RB Precision beat my first one together. I looked at him like "what the hell?". All he said was it looks worse than it is. It is probably not as great as you think. A lot of it is finish. Put a little grease or oil on it and try it a little at a time. Start it (be sure it is lined up) and back it out. If it shaves metal then it may be a problem. If it is finish then it is just a matter of working it in. Be sure that the upper has chamfers on the lug.

Doesn't Robert also work at RRA?

Quoted: Doesn't Robert also work at RRA?

I think that he did for a while but am not positive. If not he definitely know's some people there. He is working at another place now.

pliftkl

Robert at RB Precision would be the guy to talk to if you still feel concerned. He would probably know of any REAL issues you might be facing. I am sure he would help you thru the fitting. He isn't around this weekend tho'. He went to sell at a show in Indianapolis this weekend.

rra lowers have high rear shelfs, i dont use rra lowers.

I've see a few uppers lately that have been filed on the back corners of the take down lug. Must be to fit in RRA lowers.

Quoted: I've see a few uppers lately that have been filed on the back corners of the take down lug. Must be to fit in RRA lowers.

More likely they are afraid of the tight fit. I personally welcome it. They fit tight enough to make you wonder. It is a tight tolerence they hold in that area and the finish make it seem overly tight. but it keeps the upper tighter later down the road.