First, perfectly machined rings cannot correct for a bent or twisted rail, which can happen if the rail was improperly installed on the receiver. Second, on high recoil rifles, the scope can slip in fully machined rings because the surface contact area is low (due to the matte anodized surface of the scope tube). That is why some folks use three or four rings instead of just two. Using RingTrue Tape, only two rings are needed to keep a scope from slipping, even on high recoil magnum and 50BMG rifles.