I would like to try some Longshot that I just got with my 200 grain powder coated cast bullets. Hodgden has only fmj load for 200 grain with a starting load of 7.0 grains and max load of 8.2 grains. Where should I start for cast bullets?
You might want to try this data in this link; http://10mm-firearms.com/factory-10mm-ammo-pull-downs/underwood-220gr-hard-cast-pull-down/ (http://10mm-firearms.com/factory-10mm-ammo-pull-downs/underwood-220gr-hard-cast-pull-down/)
I loaded rounds at 6.8, 7.0 and 7.2 grains of Longshot. 7.2 gave me average velocity of 1043, extreme spread of 61 and s.d. of 21. I then upped it to 7.5 and got 1066 average velocity, 43 extreme spread and 14 standard deviation. I didn't gain a lot of velocity by going up but did lower the extreme spread and standard deviation. All brass looked fine . I think I will stay there for awhile. It was a fairly pleasant load. That was a power factor of 213. I did not test accuracy, but point of impact seemed closer to aim at the higher levels.
I recommend checking Hornady's load for 200gr XTP. Consider their start load as max for cast and reduce by about 10%. I find XTP data to be on the hot side for cast across 9/357/10mm for the powders I've tried.
I would probably, as a next step, load up 7.3, 7.5, 7.7 and maybe some 7.9 and test each batch for accuracy. Settle with the most accurate. As you have noted, the velocity difference is not huge, so dial it in for accuracy, which is what really matters anyway.
A 200 at 750 fps in the right place will do about any job needed done. An extra 250 to 350 fps is just icing on the cake.