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.
I drove by that church last week. If I remember right it was about medium size maybe capable of holding a couple hundred in the main part. It was likely another denomination church before but I can't recall which one. Generally I avoid that road as it's a pain to drive through. Narrow 4 lane very high traffic. But I had to get from the football stadium at the university audiology department to another location down on Kingston Pike.
I used to keep a 20 gauge broke down in my van in the summer months. It was my snake gun. I could pop it together in seconds. Now if a person wants to get sarcastic about gun laws {if only the 3 shell law had still been in effect in Tennessee} Of course I don't mean it I just remember that law. It took decades to finally get that one off the books.