Assuming this is a PERCUSSION rifle based on NIPPLE
start at the nipple itself. The nipple screws into the drum.
If you can get it out you can get a new nipple.
If not you might be able to unscew the drum from the barrel
Both can be ordered on line several places. I would start at Dixie gunworks ot the possibles shop, just google them
The area at back of the barrel where the drum screws in would be the chamber area.
This is wear the powder accumulates when you pour it down the muzzle, the forward open end of the barrel.
The FRIZZEN is on a FLINTLOCK rifle.
There is a lot of info here on the net in this forum and others about black powder shooting.
BP is corrisive so you need to clean thoroughly after shooting. Plain hot soapy water will work, or rubbing alcohol, or commmercial products.
Hope you have fun shooting it.