I think perhaps it is burning/expanding hot gasses most likely, which is why a longer barrel produces greater speeds.

Your 4 bore will burn all of the powder that can be loaded into it's case, inside very few inches of barrel length - even up to 16 drams in a 4 bore CF ctg. gun. If it didn't burn and produce gasses, then the velocity would be reduced from the load that did burn, wouldn't it?

Black powder is all burned/ignited in a short distance from the point of ignition however the gasses produced just keep expanding. This is why there is a peak in pressure very close to the breech, as witnessed by the pressure curve graphs.

Note, that 57% of BP charge is solid waste AFTER it is burned. It is very hot & glowing as it exits the muzzle, along with the smoke.