After 25 yrs I've decided to shoot a Swiss martini in 9.3x53 rimless. I have fire formed the brass but need help in developing a starting load. The rifle has sharp rifling & a shiny bore that slugs at .363. I have a mold that casts a 290gr plane base bullet at .364 (25 to 1) & a mold that casts a 220 gr plane base bullet .361. The twist is 16.75, so the 1.16 290gr bullet will stabilize at 1700, the 220 gr is only .9 inches long & .002 undersized probably won't stabilize much above 1200. I would like to shoot the 290gr. The case capacity is close to a 35 Remington. I can find loads for 250gr bullets in 35 Rem. Would it be safe to reduce the starting load of IMR 3031 called for in the 35 Rem. case with a 250gr bullet by 20% as a starting place for the 290gr in the 9.3. I will be shooting fixed ammo. Any help would be welcome.
Thanks Mike