I am assuming your rifle is 7mm, and if so a No. 1 buckshot pellet (.30) would be the proper size for slugging your bore. Oil the bore and seat the pellet in the end of the bore. Start pushing it through the bore and notice where it tightens up. Bump it back out from the chamber end and note if it has a full set of lands engraved on it. Since you will have to open a shell for the pellets, you will have more than enough pellets to try this several times. The counterboring itself is best done on a lathe.