I've killed deer with 50grs Goex FFF and a .390 round ball...Also with 75grs FFF and a .440 ball and currently use 80grs FFF and a .530 ball...

Remember the 45/70??? It used 70gr of black powder and back in the day bp just wasn't as powerful as what we have today, yet many a buffalo was killed with this gun...

I'd say that you could drop down to the 240gr Hornady .451 pistol bullet and 65-70grs of Triple 7 and kill plenty of deer out to100 yards...Plus, the 395 PowerBelt when throttled back to 70grs would actually fragment less than trying to push it out at higher velocities...Pure lead balls and bullets just don't preform well when juiced up...

My first flintlock, back in the 70s was a .45...I used it for deer, turkeys and squirrels...My deer load ended up being 75grs of Goex FFF...I tested it at 75 yards and the ball busted through a 2x4...That was good enough for me...It would also bust through shoulder blades and the spine as well as both lungs...

Get a few 2x4s and do a little testing, you'll be impressed what a slow moving lead bullet like a PowdeBelt will to at that range...