Well, that's a caution not to use smokeless powder. You certainly can use a substitute such as Pyrodex or Triple 7.
I'm assuming that it's a sidelock percussion rifle (not a flint lock and most likely not an inline.)
You didn't mention whether the gun was in use before it was given to you. You definitely want to be sure it's shootable. Make sure to clean the gun from top to bottom. Check the bore, be sure it's totally empty and clean and smooth (no rust, or caking). That includes removing the nipples and making sure they are clean and clear. Use a nipple pick to clear them and wash them good. When that's done, make a note of where the ramrod meets the muzzle. That will be your empty mark. If you have never cleaned a BP rifle, I can send you a PM and some detailed instructions, just LMK.
Once you're sure it's totally clean, you want to fire two or three caps (most likely #11) to clear the nipples. Then you might want to load with 90 grains of FFg equivalent (i.e. Pyrodex RS) and your 50 cal ball and mark your ramrod at the muzzle to show where one load gives you. It'll be a safety check for you to know whether there is a load in the gun or if it's empty, or even if there are two loads in there!
Then you're ready to see how it shoots.
You may want to go up or down on the powder charge a bit (10 gr) and see if it shoots better or worse.
You can also use sabots or bore sized bullets like PowerBelts and see if you like the way those shoot compared to a round ball. They should behave better, but you'll want to see for sure.
Besides you get to shoot a lot and make a lot of smoke and noise! what's better than that?
