What other powders do you have to choose from? 2400 is not one of my favorites, especially in a big, roomy rifle cartridge. IMR 4227 at 14-18 gr, (quite a range, that) have all worked well with .379 and .380 bullets of about 250gr., cast of WWt and a dash of tin to Lyman #2 alloy. Currently Im happily using a full case to seated bullet base of Trail Boss.
I have not reamed my rifle's throat yet.
FWIW, the 'convention' of .001 over groove dia bullets vs .0005 is a non-issue, once it gets swaged into the bore either is bore /groove diameter. What is a play is the bullet filling the chamber throat so upon launch and the transition, unsupported, from case mouth to bore, does not result in bullet base band cutting from the propellant gases. A close fit of bullet to throat and/or a softer alloy to 'bump up' helps all this. My .379 bullets fit nicely into a fully expanded (fire-formed) case mouth without re-sizing.