Those blunderbusses were the original Coach gun. Besides the increased speed of reloading possible, the larger diameter had some deterrent effect when pointed at a highwayman or other perp. They were also used as boarding guns by pirates,privateers and Euro Navies.
Most ran between .69 Caliber(16 gauge) and one inch bore diameter, and had bbls, oft made of brass for sea duty, between 12-16". Almost all were flintlocks, by the time percussion locks are available, the bell mouthed wonders weren't fashionable.
The usual load was about 6 "Small pistol bullets", 40-70 gr of coarse musket powder, and whatever wadding material was available. Effect was probably about what a modern 12 ga 000 load would do.