Fine looking doe
thallub.
While I do enjoy the PRB shot out of one of my trusty sidelocks, I also enjoy taking deer with my inlines.
Started out shooting BP in the late 60's when the 'ole' flinters cussed the caplock'ers the same way a few caplock'ers of today cuss the inliners.
Too, the longbow archers back then cussed the compound bow archers much the same way the compound bow archers of today currently cuss the crossbow users.
Funny how that works isn't it.
But back on topic...far as deer sized animals are concerned , they just aren't that hard to kill with anything from a .22 on up with the proper shot placement. And certainly not with a roundball, conical or sabot out of a bp rifle the size of 45cal.and up. Again, with proper shot placement.
IMO, the answer to the OP's question is simply the projectile that flies best out of his bp rifle. And the only way to tell which is the best 'killer' is to pull some range time shooting different bullets/loads and finding his rifle's 'sweet spot' for a hunting load at the probable max. distance he is going to shoot to.
If he can consistently put that chunk of lead through the paper plate at that distance with a respectable hunting charge , it's a dead deer.
In other words...my 'ultimate, jump back, 'best in the world' bullet' may not fly worth squat in your rifle.
Too, seems to me, to advise the OP of choices to use out of his m/l'er would require a bit more info from him.
What rifle and bbl. twist rate for example.