Should be able to take the breech plug out and remove the powder, then pound it out the muzzle end. I did not read all the posts so may be a reason that can't be done.
To avoid the problem, simply run a patch with cleaner on it through the barrel after each shot. I always do that when shooting a lot. You'll get better accuracy, too. In case it wasn't mentioned, after each shooting session, I take mine completely apart and wash with steaming soapy water then barrel cleaner patches and very light oil before putting up. haven't had a missfire or hang fire in a couple decades.
Since the first shot at a deer is through a cold barrel, I never fire a primer before loading. I can look and see the breech plug after I clean it is clear so I just load and hit the woods. I never worry about shooting a group. I only worry about the first, cold barrel shot, every time. If I can put that first one where I want it, that is all I worry about. I have killed antelope at well over 20-yards and elk at 210/ I shoot enough to know exactly where that bullet is going to hit at any distance from 50-250 yards and I am meticulous about keeping my rifle clean. I have been shooting the same gun for about 10-uears now and truthfully, cannot think of a miss in that time. And I am not a very good shot. I just know my equipment.
Again, if all that has been posted, sorry but probably worth repeating, anyway.
As an aside, one time at the Knight factory, a couple of us shot the same rifle 150 times without doing anything but wiping down the barrel after each shot. Not one slow fire.
I believe this goat was 210-yards and the shot was off shooting sticks. I was shooting 150-grains of Pyrodex pellets pushing a 265-grain Premiere bullet. This was on the Hat Ranch in New Mexico and we were doinga video for Knight Rifles.