“went off when it was dropped”, so loaded and cocked, safety off. I wonder what type of shotgun, and how old a shotgun, fires when dropped.
S.A. No need to wonder.
Safety could well have been on. Pretty common for the safety to only block the trigger on a lot of guns, particularly cheaper guns.
I have single shot shotgun from the 1960s that has an exposed hammer, and with the hammer down and a shell in the chamber, the firing pin is resting on the primer, and the hammer is resting on the firing pin with about 2mm of travel available.
If I whack the back of the hammer with a 1' piece of 2x4, IT WILL FIRE.
I did so to demonstrate to someone why you never wanted to carry that particular shotgun loaded. The only safety is a half-cock notch in the hammer that can easily be broken off if dropped with enough force.
(The shotgun is an H. Koon Snake Charmer .410.)
This was also the case with Colt single action revolvers from the 1800s, which is why cowboys always loaded only five rounds in the six gun and rested the hammer on an empty chamber.