It may not be the feed lips causing the problem. My own experience is that neither Bushie nor Rock River chamfer the bottom of the mag well as deeply as Colt, and the mags hit the bottom of the reciever, impeding full seating. Rock River seems be worse in this respect, but my Bushie also had this problem to an extent.
To find out where the problem is, insert the mag w/the bolt open. If it goes right in, the feed lips are the problem. If it hangs up even w/the bolt open, then the rib on the sides is hitting the bottom of the mag well. If that's the problem, just take a small fine cut file, and work down that side rib a bit.