By "nothing" I mean I won't be hunting with a pistol. I've got more long guns than I can shoot in a season.
Long story short, ....the pistol belonged to a guy that I used to roundy with back in the day, IE Early 80's. He wanted to deer hunt with it but it's always been against the regulations until this year to use a muzzleloading pistol. He died this spring and I now have the gun. I thought it would be nice to do with it what he wanted.
Heck, I don't even know if it will shoot those big Great Plains bullets well enough to use. I've got a limited number of them and they aren't made any longer. I'm not going to shoot any of them until I need to sight it in or in this case, probably chrono a couple of them. Thanks for checking your records.
Get close and use a round ball.
At BP velocities ME is meaningless. Almost all BP guns are anemic by modern standards of ME.
With a 9" barrel a 45 should make good velocity and will kill deer to 50 yards. Shot placement is going to be the key.
RBs we be much easier to shoot. Depending on design it may not be possible to comfortably shoot a slug with heavy powder charges.
For hunting with a pistol I would prefer a 54-58 caliber. A 58 with a RB makes about all the recoil one would want in a typical pistol with 50-60 grains of powder.
I have never shot a deer with a 45 pistol. But my son killed his first deer with a 45 rifle with 45 grains of powder at 40+ yards. A 45 pistol at ranges of 40-50 yards should be perfectly adequate.
When you start shooting bullets pressures go way up and problems result if the breech is not properly designed. Nipple or vent erosion (unavoidable in flintocks), recoil, possible nipples blown out and bullets sliding off the powder charge are the primary concerns. Shaking/splitting the stock is also a possibilty.
Dan