1st off, make sure you know what you're doing, or have an experienced gunsmith do the work.
If you're doing the work, make sure you measure the depth of the steel in the barrel over the chamber and don't drill any deeper than you need to. I used the chamber wall thickness of a 12 gauge tracker as my 'do not exceed' depth. I set it up in a mill and then drilled and tapped the holes accordingly.
When you're mounting up the scope, degrease the holes, screws, barrel and underside of the mount then apply a light coat of loc-tite to the screws, and mounting surfaces of the barrel and base. Make sure that your screws don't bottom out before they seat on the base.