megasupermagnum
45 Cal.
Not true. Powder is either burning right away in the first couple inches of barrel, or it never will be. I have not pressure tested black powder myself, but from what I've seen, pressure is also peaked quite quickly. Even though the powder is burning, and pressure peaked right away, the pressure does not drop off right away. You can see velocity gains to at least a 44" barrel. My own testing, with rifles anyway, have shown that a 20" gets you most of the practical velocity, 30" gains you maybe 150-200 fps over that depending on the load you use. According to Rice barrels who chopped a barrel down 2" at a time, their barrel at 44" was 130 fps faster than the same barrel at 30". So while 20" to 30" gains you a small chunk, going over a foot longer than that doesn't gain you as much. And these are rifles mind you. I would have to think these differences are even less dramatic in a shotgun, a larger bore running at lower pressure.
There is an argument that powder could be of poor quality in the past. Maybe the longer barrels allowed them to take all the advantage they could get from it, but it had nothing to do with burn. The idea that a shorter barrel is going to be spewing more unburned powder out the end is a fallacy. All muzzleloaders spit unburned junk out the end, the smoke is one such byproduct. Personally I feel the longer barrels of the time had more to do with military arms of the day, what with bayonets and all, and to a lesser degree, shooting competitions where a longer barrel may have been an advantage to the shooting style.
