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.