I picked up a nice sporter made on a Mexican Mauser action last year. It had a pictnanny rail which I did not care for so got rid if it and picked up some two piece bases and qd Talley rings for a scope I had. I could not get it bore sited. Upon some careful examination with my local gunsmith, we determined that the holes that were d/t'd for the rear bases were off center to the front ring holes, which made the rear base tilt to the right. I asked about filling those out of line screw holes and re drilling and tapping, but he took the base and machined some off so it rotated the base back into line , not quite 100% but very close. I then got a set of the Burris rings with the polymer inserts. I also got a couple of sets of their eccentric polymer inserts,, then re centered the reticule in my scope and actually got it bore sighted by playing with those eccentric ring inserts.
