The process starts with the receiver. Most factory receiver's front ring and rear bridge are not concentric to the centerline of the bore of the action (we will term this boreline) whether due to loose tolerance at the factory, warping under heat treatment, or overzealous polishing. Surface grinding the radii of the front ring and rear bridge concentric to boreline corrects this problem. Extreme care must be taken not to obliterate the serial number.
Freshly Surface Ground Front Receiver Ring
Once the front ring and rear bridge are ground true to boreline, the scope mounting holes are next. It is not uncommon for the front two holes to be out of alignment with the rear holes. The receiver is mounted on a fixture in the milling machine vise, precision ground bushings are inserted into the bolt bore, a precision ground rod is inserted into the bushings, the vise is offset till the rod is parallel to the mill bed, and then the centerline of the action is determined with an edge finder.
Once boreline has been established, the screw holes are checked for alignment. Most misalignment, unless particularly gross, can be corrected by cutting the holes true with an endmill and retapping to 8-40 size. Moving to the 8-40 size over the standard 6-48 also gives the added benefit of a stronger screw. Because most magnum conversions result in a 3.600" loading port, we lose one of the original factory holes. Two more holes are added to the rear bridge in addition to the remaining hole. We now have 5 screws holding down our mounts.
Screw Hole Tapped To 8-40
Now that the receiver issues have been addressed, a build sheet is drawn up with all pertinent dimensions.
We like to mount our scopes as low as possible for faster sight alignment while allowing the shooter to maintain a proper cheek weld with the stock. This also helps to eliminate felt recoil to the face with heavy kickers as the rifle has less of a running start. The wide variety of ocular and objective bell dimensions, tube lengths, and eye reliefs between different makes and models of scopes combined with differing barrel contours and bolt handle positions results in each set of mounts being a unique configuration.