The problem with that thinking is
A) not all barrels are thick, rifle barrels
B) the powder has mass and a can of black powder is often 16 ounces of the stuff.
So say the barrel will hold a full can of powder. Most Bess, which are thinner walled than a rifle, will hold pretty much a full can of powder....( at least I think they did when we checked on this 30 years ago..., I may have to double check)
When you set that off, lets say you only actually burned two ounces, or 876 grains of powder..., that 876 grains of burning powder then pushes on 14 ounces of unburned powder as it heads towards the muzzle.
TRUE as the action happens, as the mass moves forward the space between the mass and the breech plug is enlarged which subtracts from the pressure as the powder is burning and increasing the pressure.....
BUT that's the same as putting 876 grains of powder down the barrel followed by a wad and 14 musket balls. Do you think the barrel will contain that pressure, because the burning powder and the pressure from that will extend beyond the thicker part of the barrel....
Which is why we never actually tried this in a musket barrel as nobody wanted to replace his Bess barrel...IF it failed....
LD