John: Mauser actions have a lot of good points but they are not particularly rigid. I bed them with forend tip pressure - about 5 to 7 pounds of weight required to pull the tip away from the barrel. The previous suggestion to put some shims under the barrel at the tip in varying thicknesses is a good tip to see if it helps.
In a heavy caliber such as yours is long shot string groups aren't nearly as important as getting the first 2 or 3 in the right spot.
luck with it.
