Slims

Practice w/ balls and patches (i.e. the cheapest stuff around) since they are plenty accurate and cheap. Use a good lube like Bore Butter. For hunting, get some nice sabot rounds (and practice a couple times with them too).

Clean your gun religiously, and be super anal. Otherwise, you will go to shoot it one day only to find that the bore/nipple has rusted. Get Hoppes blackpowder solvent.

Get a range ramrod. The best kind is solid brass with a nice handle. The dinky ramrods that come with the rifles just aren't practical for repetitive shooting.

After you get get the hang of things, load the gun and put a mark on your ramrod(s) where they should be roughly parallel with the muzzle of the gun. This way, if you forget and either double charge your rifle or load it twice without shooting it, you will be able to tell before you blow your gun up.

Don't use blackpowder. Use pyrodex instead. It is more consistent, and much less corrosive. Make sure it has the right F rating.

Get one of the old guys at the range to show you what's what.

Edit: And my favorite deer caliber is .270, but .50 blackpowder is nice too.

Good luck.

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