Truing up the face of the receiver on a Mauser action requires some specialized equipment.
First, secure the receiver in a vise and use a depth micrometer to measure the distance between the torque shoulders in at least three different places.

Using a mandrel will help us true everything up. Screw the mandrel into the receiver and secure it in a three-jaw chuck. Use the live center to support the other end of the mandrel.

Now you can true up the face of the receiver, running the lathe at low speed. A light cut is all that is necessary—just where the barrel will touch.

Now measure it again, the variation should be less than one-thousandth of an inch.

Now there's a job that's a lot easier if you've got the right tools.
-- Larry Potterfield