http://www.aboutguns.net/mcmillan.html

Posted: 09-22-1999 10:29
When an action is made the scope mounting holes are drilled prior to heat treating the action. All actions warp to some degree during heat treat which means the holes which were true when drilled are not when you mount the scope . A one piece mount will more or less seek a medium and be more true than two piece mounts but can spring the scope tube when you clamp the scope down. This is the cause of 90 percent of scope trouble. That is why knowledgeable smiths lap the rings in after the rings have been installed. I prefer two piece mounts that have been lapped in

Hope this helps.