I read a story back in the 1970s where a guy bought two blunderbuss replicas from Dixie Gun Works, kept one with the belled muzzle, sawed the other one where the straight cylinder bore ended where the bore started to bell. He got the same patterns from both guns. Once the shot charge leaves the cylinder part of the bore, the bell has no effect on the shot. The writer concluded that blunderbuss muzzle is belled so that it can be easily loaded on a rolling ship or rocking stagecoach.
Shooting at an outdoor range (I believe it was an abandoned gravel pit--I'm relying on memory), he also loaded the blunderbuss with shot, and tried the usual pawn shop talk loads: broken glass, nails and scrap metal, and gravel. He fired at steel coffee cans (1970s cans mind you). He got penetration only with convention shot or from cartridge casings accidentally included in the scoops of gravel. Random shaped scrap metal and gravel dented the cans but did not penetrate.