Well, I got paid from my latest business trip, and have the itch to drop a couple hondo on a new blackpowder rifle, or even a smoothbore. I’m a pretty big shooter, but I’ve just never had the chance to play with blackpowder.
So, besides the gun, what else do I need? I’ve already got a cleaning kit for my other guns. Do I need a measuring kit? Powder flask? Caps? Anything else?
Tripler
I really want to give it a shot.
I only have blackpowder revolvers, but I’ll take try…
You’ll need a powder flask. You should also have a measuring tool so that you A) get the correct amount of powder into the rifle; and B) are not pouring powder directly from the flask into the barrel. (That could cause a nasty explosion.) If you’re gettig a percussion rifle, you’ll need percussion caps. If you’re getting a flintlock, you’ll need flints. You’ll need balls or Minie balls. Patch material and a patch knife. Lubricant. (There are purpose-made lubes, but you can also use Crisco.)
You also need to be prepared for a very messy clean-up after every shooting session. Very messy. Your cleaning kit should be of the correct size. There are hose devices available so that you can clean the barrel more easily.
One of these days I’d like a Lyman .50 Plains Rifle (‘Hawken’).
I was going to say ‘take a stab’, but we’re not talking cutlery. 
I forgot to take out ‘take’.
My parents were huge black powder muzzleloader shooters when I was a kid (they both won the PA state championships several times, I think). I’m trying to dredge up my memory.
I know they used powder flasks (or horns…the PA black powder ml crowd tends to overlap with the pioneer/Revolutionary re-enactor crowd), a tapper (a small wooden hammer-type thing that pushed the ball/bullet down far enough that they could fit the ramrod in), strips of pillow ticking or bedsheet to use as a wad, percussion caps (or flints…like I said, the crowds tended to overlap and there were usually both flintlock and percussion events), a small knife to cut the wadding off, the ramrod itself, of course…
Depending on whether you intend to make your own balls/bullets, you would need a mold (or two, if you want to shoot different types of projectiles) and a lead furnace (my dad had a tiny electric one).
You’ll need different stuff to clean BP guns because the residue is water soluble not oil soluble. Hot soapy water works great but you’ll get rust unless you dry completely and oil after cleaning. I avoid all that by using an oil called Ballistol that mixes with water. Mixed one part oil to ten of water it makes a moose milk looking concoction that quicly cleans all the fouling and doesn’t cause rust. I even spray it directly into the actions of my revolvers and lever rifles. I even use it straight as a lube and to clean smokeless guns now. It was developed in Germany at the end of the 19th century since it take water to clean the residue from corrosive primers. Modern primers aren’t corrosive and neither is black powder residue aside from attracting moisture but black powder substitutes like Pyrodex are corrosive.
Get a capper, I think Ted Cash makes the best ones. Properly seated caps are key to reliable BP shooting and pressing them on with your thumb is somewhat dangerous as there is enough energy in the cap to blow you thumb apart shold one detonate while seating. Cabelas carries them as well as Ballistol I think. It is a messy sport but a hell of a lot of fun.
I’d reckon you’ll need a fringed buckskin jacket and a beard. Oh, and all that stuff the other folks mentioned