338/300 ?


Well-Known Member
Apr 12, 2002
Loon Lake WA
Why are they using the 300 UM to make a 338 UM, instead of just using the original 338 Ultra Mag? Don't get it? Thanks for the explanation!


Well-Known Member
Dec 15, 2001
They do it because the 300 RUM case has more case capacity than the 338 RUM case.


Because of the length of the bullets used in the 338 they had to shorten the case by .100 thou.
If you want to use the 300 ultra case and neck it up to 338 you might onyl have a single shot on your hands as the bullet lengths will dictate whether or not you can use your action as a repeater. Optimum conditions are when the onyl part of your bullet that protrudes past the shoulder/neck intersection is the boatail of the bullet giving you maximum case capacity.
you will have to experiment with how much freebore you have to determine where your bullet will actually be seated.
but if you have the dimensions of the bullet and dimensions of the case then you can design the case with the bullet and have a reamer made to match those conditions.

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