On the Discovery channel a while back the program "Mythbusters" were testing to see if the old cartoon gag of lighting a trail of gunpowder would explode a moving keg of gunpowder. You know Bugs Bunny cartoons. Well they succeeded in doing it.

Where this applies to this discussion is that they were using only a 5 gallon keg and did not fill it completely full to mimimize the explosion. The explosion was quite dramatic even at that. So I can imagine what 63 gallons of black powder would be like. It would certainly be an extremely large explosion. But without some sort of addition casing other than a wooden barrel, wounds from projectiles would be minimal. Therefore its primary effect would be blast and burns.