Jim - your TBoss loads will probably only run you in the 900 to 1,000fps range.

I suspect the bore is probably normal, ie: .366"???? This, of course makes for every deep grooves - which is the same scenario as my 9.3x57 Husky. It has a .370" groove diameter, .356" bore, giving .007" groove depth.

If yours has a typical .366" bore, then it's rifling is .075" deep. My 9.3 shoots amazingly well with undersized .365" and .366" bullets - under 1/2" & up to 3/4" with most bullet weights.

Perhaps you can try a few normal loads with jacketed, just to see how it shoots and save those for hunting, relying on cast for plinking.