A bit of artillery history

Rodman's Great Guns
Including some innovation on gunpowder:
Perhaps even more important than his casting procedure was Rodman's development of progressive-burning powder. When any gun fires, of course, the volume of the bore behind the projectile increases as the projectile travels toward the muzzle. The normal black powder grain, however, irregular in shape, burns from the outside, so that its burning surface area continually decreases. Thus, in a normal black-powder piece, initial breech pressure is the highest obtained; the forward traveling projectile increases bore volume as the powder burns at a decreasing rate. Both occurrences reduce interior bore pressure.

Rodman proposed powder pressed into hexagonal grains perforated with several longitudinal holes so that as individual grains burned both inside and out, albeit almost instantaneously, the burning surface of each grain actually would increase. Rodman's powder didn't increase pressures--it simply maintained a higher bore pressure than normal powder could, as the projectile traveled forward. The result, logically, was an increased muzzle velocity of the projectile.