Doesn't matter. It still has to do with the exterior surface area of the bolt head. Even if the interior dimensions are smaller, that pressure is still being exerted over a larger area with the Lapua which puts more stress on the bolt head/locking lugs. They have been built on regular actions and ended up having problems with lug set back. Ask Sharpshooter about his experiences in building them to test them out. He said in a thread on here that it wasn't a good idea and explained why. I'll see if I can find the thread. He built one on a regular long action, small shank. He had lug set back and the chamber was temporarily swelling during firing and springing back due to the chamber walls being too thin which caused cases to stick. The RUM is smaller in diameter so the chamber walls can be made a little thicker and has a rebated head which reduces the amount of bolt thrust with the other benefit of it fitting a regular magnum bolt face. If it was safe enough to do on a regular long action then Savage would have done it. The RUM is about the biggest that can be safely put in a regular action.