Looks like the live fire accident last night was a mortar round detonating inside the mortar tube:

At least seven Marines are dead and another seven are injured after an accident Monday night in Nevada in which a mortar round exploded inside an artillery tube, military officials tell NPR’s Tom Bowman.

The Marines were taking part in a live-fire exercise, those officials say. “Shell fragments, I’m told, killed almost three [Marines] immediately,” Tom says. The others died before they could be evacuated to a hospital.

npr.org