I will still stand by my suggestion, and my reasons will be, that folks that STORE powder do so in a place where powder is safe and not exposed to a lot of changes in temps and moisture issue's
folks that LOAD inlines and muzzle loaders(and I base this on yrs of dealing with customers in my gun shop)
have a tendency to not store guns as well as powder
I have seen many bring a COLD gun into a HOT room, and condensation can form and alter powder, they have possibly carried the gun in the raina nd snow and possibly got some water down the bore which again can effect powder
it can also cause action of corrosion like build up on the bullet in the bore!
I have seen loaded guns stored in barns, sheds, trucks, and all over again exposed to many weather like changes and humidity and ??
add in YRS of this, and that powder can be BAD , and firing it might just be a fizz and and just lodge the bullet in the bore in a different place
the SAFE and propper way IMO< is have a gun smith remove it safely
so OP doesn't have to experiment doing so themselves,a s its obvious to me they DON"T know how to, or wouldn't be asking here
folks with more experience maybe can handle doing so themselves.
but suggestion to a a new owner of a unknown gun storage or history,(and again possibly how or when loaded and with what 100% for sure that is) to just SHOOT IT, doesn't sound very safe to me!
and to the OP< how do you even know for 100% what powder is in the bore?? maybe this question should be answered here before we give suggestions on firing it too?