A shrapnel shield is one way to make something NOT a pipe bomb. Basically, it is a system for allowing gasses (pressure) to escape easily in all directions (so it isn't a combustion chamber) while stopping any larger chunks from leaving (or only allowing them to leave in a predetermined direction.

The dynamic Newton meter has a shrapnel shield consisting of a large loose-coil steel spring. In case of a failure during a test it lefts gasses out on all sides but only lets the engine tube exit straight upwards.