A slight over charge, with a fouled barrel and too tight of a projectile short seated tryinr to get off a second shot at game, and you just made a bomb.
I don't hardly think that's right at all. If you slide a projectile down the barrel and got it even close to the powder charge, you might ring the barrel, but the simple act of loading the projectile is going to clean alot of the fouling out from the muzzle down.
Sam Fadala and Lyman have tested and tested and retested tremendous powder charges and multiple balls and such in an effort to blow muzzleloader barrels and they have bulged a few but never turned one into a bomb.
Even in the Winchester 1876 tests they had something on the order of 2500 grs of powder and a handfull of Martini bullets in that barrel and only managed to bulge the reciever.
To make a bomb you would have to solidly plug both ends and not allow for anyway for the gases to excape except to split the material it was contained in.
There may be some cheap Spanish and Brazilian made muzzleloaders from the 60's and early 70's that could be made to come apart , but there again it would be someone rushing the threshold of Darwins theory.