On a small scale, start a good bon-fire in your back yard and then pitch in a couple of .22 rounds. They will soon ignite and burst and, in all likely hood the only thing leaving the fire will be the shrapnel. The bullet will melt. Same applies to larger cal. cartridges. IF a casing is against a bullet when the case goes it may propel the bullet away but not necessarily in the direction in which it is pointed. If there is a very small amount of powder in a horn/flask with a snug fitting plug, the container may do just that, contain the explosion and propel the plug away from the container. But, then the question is, If the container was in fact tight in itself and the plug was secure in the spout, how is the contents going to ignite? Best practice is to religate the powder away from any source of ignition; intended or accidental.