Originally Posted by Eastcoastshooter375
I have seen ample supply of 7mmRUM brass but 300 is limited. I know the 338 ultra has a shorter body, but wasn't sure about the 7RUM.
I would expect the neck thickness would be off, but how much, I'm not sure.
When I tired out my 7rum I went to 300 rum and necked the brass up I already had in 7 rum. I've got over 100 rounds of this brass in use and the only issue I've had at all is a couple of necks splitting on the first firing. Annealing before necking up would probably stop this. For me it was simply staving off dropping a $100 bill on brass. As to neck thickness, it'll be a smidgen thinner but who cares; I'm still getting .5 to .6 moa from a 9# 300 rum without a brake.
Do a 50 and see if it works for you.
BTW, Lube the inside of the necks a bit so they neck up more evenly.