Re: 300wm brass
NO! The 300 Win Mag case is longer than either the 7mm Rem or 338 Win Mag. Case length on both the 7mm and 338 Mags is 2.500", while the 300 Win Mag measures 2.620". You'd wind up with a case neck only about .140" in length, far too short in a case that's already considered to have a very short neck. You can do it the other way and form the 300 down to either the 7mm or 338 (but I'd suggest that you'd have to be really, really bored to do this) by bumping the shoulders back, necking up or down as the case may be, and then trimming to length.
This stuff isn't exaclty uncommon, or hard to come by, even in the crazy times we have right now. I'd save the case reforming for more extreme instances wherein the case in question can't be procured in any other manner. Trust me, it's a last resort.