I have a lot of 300Win Mag case. How are they for making 338Win Mag cases by just putting them in the 338Win mag resizing die???? Also, how about 7mmRem mag case doing the same?????
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Problem 300wmag is 2.62" the 338 and 7mag are 2.5" so you will need to trim about .12" less from the 338 more for the 7mag. Plus the old 300Wmaf shoulder neck junction will now be in your new neck.

I'd sell it and buy some 338 and 7mag.