Could have even been a break down single shot I hope not though.That would mean at least 5 chances to disarm him. Six people shot and if I read right more shot at. Now even with the plug out most hold what? 5 shells? Meaning he had time to do a reload too most likely. In close quarters the shot would not scatter that much.
Probably not break action. They would have tackled him sooner. A pump with a 20" barrel be 8 shots without reloading, IIRC. Even the 18.5" Mossberg Cruiser model holds 6 rounds, including the one in the chamber. Of course after reloading you either have to top off the magazine or live with one less round on that go around.
Considering we are talking a shotgun here, in a largish room, he could easily have shot them all before reloading, even with the 6 shot model.