Many "old timers" would determine how much powder was the proper load for their guns by firing it over a white surface such as a sheet, or a snow cover. When they began to see the black specks of unburnt powder on the snow they would know that was the maximum amount of powder that their gun could effectively use. That loading any more in the shotgun, rifle or musket was a waste of expensive gun powder.

That is what makes me laugh about our so called modern muzzleloading hunter who are pouring 120 or more grains of powder down a 50 cal rifle with a 20 inch barrel length, The gun only needs about 60 to 70 grains to preform properly. Anything thing more is simply wadding, because they are blowing the extra powder out the barrel. But they think the more the better. LOL.... now I will give you that the extra "Wadding" does increase the presures within the barrel. That is why the barrels will blow up.

A black powder weapon will only burn the amount of powder it needs to get the obstruction out of the barrel and the lenght of the barrel will determine the amount of powder that is needed to do that. Hence the "Old Timers" addage "the longer the barrel, the hard a gun will shoot" and "Long Tom" for a guns name. That is not true with modern smokeless powder which does burn almost instantly.

The same is true with a barrel of gunpowder. The explosion will only use that amount of powder needed to build the pressure necessary to burst the Barrel.