Anyone try installing a Colt AR upper on a RRA lower???

[Silent Bob]

I was wondering if anyone here has tried installing a Colt small-hole upper on a Rock River Arms lower. If so, did it fit all right? I have heard that it can be difficult installing uppers from other manufacturers on RRA lowers (too tight fit). Thanks in advance.

 

[MiniZ]

There are RRA lowers that have a "shelf" that is a little too high. It is not a universal problem for all lowers.

While not Colt specific, I had to give an Oly upper about three swipes with a file on the back lug to get it to line up.