2-1/2" case in a 20 bore might hold roughly 120gr. 2F for a heavy charge. The rest of the area is comprised of cup-wads and over powder wads. This will, in the enclosed system of a ctg. gun, give approximately 1,800fps with a 320gr. (or thereabouts) ball. Any conical will travel very much slower, depending on the weight, probably 1,300fps to 1,500fps. The rate of twist is the deciding factor.