The Lyman Cast Bullet handbook and 48th shows 248gr-249gr cast bullets at 1900+fps in their 375Win data in a 20" barrel, the 28" H&R barrel should easily do 2000fps with the right powder. I chose Reloder7 for mine with the 270gr Mt Baldy as it should work good in the longer barrel, but haven't been to the range yet. They show RL7 27.5gr start and 32gr max compressed for the 249gr bullet at a COL of 2.450", WLR primer, start low and work up. Their 38-55 data for the same bullet starts at 24gr, max is 27.5gr, but COL is 2.550".
Tim