This may be a dumb question, but can 338 ultra mag brass be resized to 300 ultra mag? And if you can, is it just a matter of running it through a 300 ultra mag resizing die? Thanks.
The reason I am asking this question is that I have a 300 ultra mag, and like everyone else, I am having a hard time finding brass. But I know where I can find 100 pieces of 338 ultra mag brass.
That's a great deal!.I was at a gun show and someone had nosler custom 300 rum in 165 gr partition for $40. I bought the 2 boxes he had for the brass
The usual 300 RUM neck length is ~.306Since the shoulder on the 338 is shorter than the 300, all you need to do is treat it like the false shoulder dasher guys do when they form their Dasher brass from 6br. When you neck the brass down, leave a false shoulder to head space on. I also like to jam the bullet.
I am not sure how much the short neck will affect things. i would watch for a carbon ring in the throat area.
The pic is a prepped 6 BR and ready to FF to dasher.
The usual 300 RUM neck length is ~.306
Forming as you suggest would result in a neck length of .216 or less.
While there are designs with this sort of neck length most are not 30 caliber and none are as large as a RUM.
It will work but will it work "fine" well.....
As you point out, carbon ring, possible chamber erosion in the neck area.