Take a piece of pvc - go crazy with the diameter, but make it 2" long. The pvc, filled with bb's will be sealed at each end by the two part foam mixture (used by some boaters to fill a hole in the hull, keep buoys floating, etc). The bolt will be designed to extend the whole way through the pvc shell ejecting the bb's in a wonderous cloud of death

. the bolt could be pneumatically powered or spring powered. (just a thought)
*edit the foam is called Urethane foam.