Precision Firearms upper to tight to fit lower RR lower AR15

[soddyjohn]

I purchased a Precision Firearms (6.5 Grendel Neptune 5 ) a couple of years ago and put it on my Rock River Varmint AR15 and fit up fine. Due to health problems I sold the upper to a friend. The uppers' lugs would not fit or even insert into his Rock River lower pin area. His lower was a new Rock River lower unit. I removed just enough off the side of each upper lug until it inserted into the lower unit. Very tight fit and hard to lock up. He took it home and tried to put it one several other (inexpensive lower units) and it you not fit any. The upper lugs are to wide I think.
So my question is what is the best way to fix this (take more off the sides of the upper lugs or what?

Thanks