As Waksupi said, it COULD be an SEE. It is a real thing, but not normally encountered in muzzleloaders. Now if you are using powder sold in a paper sack from the back of a truck in Chinatown you might well have an issue like that. If you are, you are very lucky.
We are all just sorta guessing based upon our individual experiences. Ultimately the responsibility is yours and I think we have an expectation that you are doing your part, i.e.: reading the fine manual, using the correct materials, using the proper procedures, etc. The only home-made powder you should be using is that which you made yourself. That way, you know exactly what it is. If you blow your gun up with your powder, it is your fault. I learned this lesson shooting someone else's handloads.
If it doesn't have a manufacturer's label on it, you don't want to use it. If it has a label but you never heard of the manufacturer, don't use it... don't even BUY it. I would use Pyrodex or Triple 7 if I could not find black powder in the original container for sale. In these times of the Internet, you can order powder easily from your own desktop, so there is no reason to use questionable powder.
I had a friend once who was then tasked with disposing of gunpowder seized in police operations. His department got some powder from an illegal fireworks factory. He poured the line of powder out and touched it off. Well, the powder went off... and took two of his fingers. It could have been much worse. Most likely it wasn't actually gunpowder, but we'll never know.
I would suggest getting some powder that you know the source of. It needs to be actual BLACK POWDER or a legitimate substitute. Brands to look for are Swiss, Scheutzen or Goex. Elephant will work. There are a few legitimate substitutes that some use like Pyrodex, Triple 7, and others that I am not familiar with. Load the gun up again with stuff you can be sure of and see if the double boom recurs. If it doesn't, then dispose of that stuff and never use it again.
Not all gunpowder is Black Powder. If you loaded your gun up with Bullseye and fired it, they might never find pieces of your gun. You won't be able to help with the search because you will quite possibly be blind and maybe dead. Smokeless powder is NOT for muzzleloaders... though I understand that some idiot designed one that can use some smokeless powders. That ain't your gun though.
