Here is a site of one of the salute gun manufacturers. Take note that they guns are made up to 30 mm in bore, Also the suggested load for a 16 mm, about 20 ga, is 240 grains of black powder.
I have seen in the videos that the powder is extremely coarse grained, perhaps a little larger than out cannon grade. On top the powder, they load a cork, which is so oversize that they require a loading stick and mallet to get the cork down the bore.
The short barreled rifle stocked models are made up to 40mm in bore. They also use thunder mugs in some locations. The German thunder mugs are made up to 100 mm in bore. That is roughly 40 inches. The suggested load, over a half pound per shot. And for those who wish to break eardrums and windows, there are cannons of very large bore.
The one pictured here is a breech loader that fires a percussion cap into a hole in the center of the cartridge base to ignite the powder.