The smooth-bore arms used shot--buckshot in this instance, with between nine and twelve pellets for the most part, or large round "pumpkin ball" slug, or with a mix of sizes like the "buck and ball" load.
Historically, soldiers equipped with a smooth-bore .69 caliber musket (a rough equivalent of a 16-gauge shotgun, while the old Brown Bess/ India pattern English musket is roughly twelve gauge, or even a ten-gauge in the earlier examples--recall that the balls would be very undersized, so the U.S. martial musket .643 or .65 ball was "19 to a pound" or 18 or 19 "gauge" for use in a larger bore...) would have received an ammunition allotment of 1/2 single ball cartridges and 1/2 "buck-and-ball" cartridges. There is a description of smooth-bore armed New Jersey troops awaiting combat at Gettysburg making their own all buckshot cartridges from their issued buck-and-ball ammunition.
A sentry often used an all buckshot cartridge, particularly in instances where lickspittle officers would not allow the musket to be "unloaded" merely by firing or discharging the piece in a safe direction. The alternatives were a single ball cartridge, without ramming the ball so that it was "loose" and could be extracted easier, or a buckshot cartridge in which the wadding could be removed with a ball puller, and the shot and powder poured out of the barrel. Having to dig out the bullet of a rammed single ball cartridge would have presented a more onerous proposition.
A double-gun in the hands of cavalry or bushwhackers or partisans/guerrillas would have been a formidable multi-shot weapon indeed. The only problem with many of the arms "brought from home" in addition to the non-standard, non-military calibers complicating logistics was that frequently civilian arms were simply not as sturdily or robustly constructed as even the most pitiably out-of-date/ obsolete smooth-bore.