I have not used 300-MP personally
Alliant web site list load data with 350 grain Speer UCSP
Fed 215 primer, OAL 2.25" 49 grain (max) 300-MP

Alliant Powder - Reloader's Guide

I have used H110, N110, 2400, A9, Lil"gun, IMR4227, N120, and W296, load data out of the Hornady Manual.

Additionally I have used IMR Trail Boss and several other IMR powders as described on John Ross's web site.

I dont see that brass headstamp makes any difference. Starline , Hornady and Winchester brass I measured all had the the same volume I measured, around 63 grains of water if memory serves.

All the brass I have is for Large rifle primers not the original large pistol primer brass and is marked accordingly.

Good luck and be safe
Ruggy