text:  Another method of increasing firepower was to vary the <unk> 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 <unk> of the black powder explosion also meant that guns were subjected to extreme stress . One important step was to press the powder into <unk> , allowing a slower , more controlled explosion and a longer barrel . A further step forward was the introduction of chemically different brown powder which <unk> more slowly again . It also put less stress on the <unk> of the barrel , allowing guns to last longer and to be manufactured to <unk> <unk> . 
  