I will second the advice above from kamlooky. Most rifle manufacturers have a hand polishing or buffing step before bluing, so no guarantee that the receiver surfaces are dead parallel, or to exactly same height differential. Often, this is a major issue on milsurp receivers used to make sporters. It can be a real eye opener to set a straight edge on one installed base and carefully slide the straightedge across to the other one. It is a good practice to verify by starting on second base and checking back to the first one. Shims and epoxy to build up or straighten tipped bases, or really fussy scraping and filing to the bottom of a base.