In the spirit of conversation rather than why can't I make this work .
I have been working on a 264 WM 1st there was misinformed brass conversion. it seems that a 300WM is not only longer but has a fatter flatter shoulder also,and the 275 H&H dies really didn't help much as a transition die...
More good stuff this 50 yo rifle has been apart and together a lot but doesn't seem to have been fired . So among other things there is no throat wear which makes getting the patch into the throat and still covering any of the nose/bore ride and not have half the 258-120 sp down inside the case requires the patch to be back off the nose/ogive about .2 inches to get even close to a book OAL . The bore ride is of sufficient diameter to make full contact for that distance. This and helping another member, Over Max ,I have been back to read the guidelines for success .....several times. I have 4 rifles that have delivered the goods,1 that amounts to slap some paper on it and shoot little groups with whatever powder was laying around. I have 2 to work with now that just don't have the throat space to get the patch in the bbl. There was a thread that PDawg,I think,did working "the half patch" with little to no joy and mediocre at best results. For me getting into the high 2000s and minute of Mulie to 300 yd would just tickle me silly.
Anyway the 264 remains unfired today but there are 20 rounds that will chamber and eject in 1 piece and the lube dents should come out of the 8 survivors in the 300WM RP an 12 brand new Hornady 7mm RM simply chamber necked and loaded with a cast start load to get the cases out to the right shape. On that note 12 gr of Unique is a lot of powder (to me anyway) but holy cow the LCHB lists 23.? as a max load . That struck me as a 25k load . I only loaded 27gr in 1 1/4 oz duck loads back in 80s when it was still ok to kill geese dead with 4s at 60 yd.... oh well.
Yes I'm well aware that this plinker load isn't the fast track nor does even tip toe into the "cartridge potential", but then my current concern is to get it on paper and polish the bore and form a few pieces of brass in preparation for the real loads and a full case of something slow. With just a little luck it might even let me get enough slicked up to get a little more patch in the bbl.