I'm not a professional gunsmith, far from it, but I have built several muzzleloaders myself. After the gun is finished I "proof" it by firing an extra heavy charge. Here is how I do that.
I don't do a double charge of powder as some have told me to do, but I do use about 1 1/2 times a normal load. So, for a .50 that might use a 70 gr. charge of FFF I will load it with about 100-110 grs. With a .54 that I normally load with 90 to 100 grs of FFF, I will load it with about 140-150 grs.
Just one patched round ball.
Then, I set the rifle inside an old tire that is lying on the ground. I put the rifle butt against the inside of the tire, and rest the forearm on the tire side with the muzzle pointing at a good backstop.
I lay a sandbag on the barrel to help hold it down on the tire.
Then, I tie a cord to the trigger, and get far, far back away from the gun, at least 20 yards back. I then pull the cord to fire the gun.
If all looks well after a careful inspection, then I'll load a normal charge and fire the gun from my shoulder.
NOTE: I do this with rifles. I have not tired it with a smoothbore. Smoothbore barrels are thinner than rifle barrels, so I would be very cautious with a fusil. With these, I don't really "proof" them. I just start with a light load and work up.