I don't post much, but read alot on the site and have a question.  I have a "post ban" Colt competion 16" H-Bar that I've had since '96.  My question is will a RRA upper fit on my lower.  A friends DPMS upper fits on my lower and my upper fits on his lower.  I've read a few threads about the "unique" holes that Colt has used in the past, and I'm nervous about buying an upper and that it won't fit when I get it.  Any links or info will be greatly appreciated.