From what I've read, AR uppers are, more often than you may think, manufactured with the bore for the barrel on the tight side to provide tight barrel fitment. This is done to help ensure accuracy. To install a barrel in a tight upper, the usual practice is to thoroughly freeze the barrel (to shrink it) and warm up (not roast) the upper (to expand it).
I have never been personally blessed with an AR upper that was made to provide improved accuracy, so I've never performed the above noted freeze and warm to fit process.
Best of luck.
MHO, YMMV, etc. Be well.
ETA: Beat by a quicker typist.