I would try it with the A-Max bullets first and see what happens. You can always set the barrel back if you have to but you may not have to.
The API projectiles, pretty much suck for accuracy to be blunt. If you get 1 moa with them you are doing AMAZINGLY well with these bullets.
The 750 A-Max is generally extremely accurate in its preferred loads. If you have a 1-15 twist, look to load it to 2500-2600 fps with H-50BMG and generally accuracy will be up to what the rifle is capable of.
Again, I would see what the A-Max bullets do before investing in a reamer and setting the barrel back.
Allen Precision Shooting
Home of the Allen Magnum, Allen Xpress and Allen Tactical Wildcats and the Painkiller Muzzle brakes.
Farther, Faster and Flatter then ever before.
Web Page: www.apsrifles.com