Rifle and musket powder are much finer grades of powder than artillery powder or the powder used in shells and do have different burn rates. The finer grades such as priming(4F), rifle and pistol(3F) and musket powders(2F) generally burn much faster because the granulation is smaller with the more "F's". But the principal still remains that to burst a Hogsheads Barrel probably would not reguire more that two or three gallons of powder under the right containment. So you would still have the other 60 gallons (which I know is not proper way of measureing, since we are using volume instead of weight as a measure) of powder which would be scattered and ignited by that initial explosion.