Another method of increasing firepower was to vary the projectile fired or the nature of the propellant. Early ironclads used black powder, which expanded rapidly after combustion; this meant cannons had relatively short barrels, to prevent the barrel itself slowing the shell. The sharpness of the black powder explosion also meant that guns were subjected to extreme stress. One important step was to press the powder into pellets, allowing a slower, more controlled explosion and a longer barrel. A further step forward was the introduction of chemically different brown powder which combusted more slowly again. It also put less stress on the insides of the barrel, allowing guns to last longer and to be manufactured to tighter tolerances.