First thing to look at is the muzzles. To properly load a patched round ball, the crown should be nicely radiused and smooth. You can do this with a tapered stone bit for an electric drill. Wrapp find emery around the stome and rotate it back and forth with your fingers, not under power. Chenage the mery to finer and finer grades until you are using crocus cloth (red). Your thumb with a final polish with crocus cloth will also do a very good job if you are handy with this sort of thing.
: The reason is a sharp angled crown will cut the cloth patch and result in very bad fouling and poor shooting.
: Next, is to scrub the barrels inside with 0000 steel wool to smooth and polish the lands. Wrapp the (4-ought)steel wool around an undersize brush or jag and oil it, then run it up and down, maybe 20 or more times. This will make her smooth and ready for shooting. It should be a one-time thing, but should be done, especially with Spanish and Italian guns which are know to be a bit rough, sometimes. It will not hurt the barrels.
; Colorado's suggestions are very good for learning to shoot this rifle and finding what it wants. If it is a true .50 cal, that is, with a .500" bore, and .516 to .520(or so) groove dia, then use a .495" ball with a .015 to .018" patch wet with lube. Use spit, Hoppe's #9PLUS or some other BP lube. Bore butter has been known to leave a kind of fouling or coating in the bore that is said to be difficult to remove later.
: I personally use 50:50 Murphy's Oil Soap/Castor oil or Murphy's Ol Soap/Neetsfoot Oil (not compound) 50:50 mix and these work wonderfully.
: For an easier loading combination, a .490" ball and an .018 to .022" patch might work. I use denim bought by the yard from WallMart or other sewing outfit. I take my claiper swith me for measuring. The cloth must be washed, preferrably twice. I use an Arch punch, or large ball bearing and a bearing race for punching out patches (with hammer). The ball and bearing race works well, and you can make a hundred patches in 10 min.or less cutting up to 3 thicnesses at once. The Arch Punch is faster, but costs more. Balls and races can be found for free at large machinery or large electric motor repair places. I use a 1.625" ball and a 1" race for .45 and .50 and 1-1/4" race for .54 and .60cal. A 1 1/2" Arch Punch is used for the .69.
: You will generally find better accuracy with real black powder, rather than one of the phoney powders like Pyrodex, BlackMag3 or T-7.