There would of course be shotguns in the "linear warfare" era, which I presume means 17th-19th c. In the 18th c. they'd be called fowling pieces, or just fowlers, because like modern shotguns they were effective for hunting flying birds. Just like now, of course, they were rather short range. Even more so, actually, because of the lower velocity they'd get from black powder, and they would not have choked barrels to keep the shot pattern more compact.

But large-diameter shot ( like 00 buckshot, around.30 caliber) would travel reasonably far, up to around 100 yards. That'd be near the same effective range for a regular smoothbore musket shooting a round ball, and so "buck-and-ball" loads were not uncommon: one musket ball with a few-to-several pieces of buckshot sitting on top. George Washington and General Henry Dearborn thought them useful during the Revolutionary War. Washington suggested the troops load their muskets with one ball and four or eight buckshot, "according to the strength of their pieces". Later, Brigadier General Thomas Meagher also liked buck-and-ball loads, his 69th NY "Irish" Brigade in the Civil War long kept their 1842 .69 caliber muskets just for that purpose. Whether the longer distance accurate shooting with the Miné bullets was more effective I am not sure of: but of course later the faster rate of long distance accurate fire that came with the advent of breechloading weapons certainly was more effective than buck-and-ball..

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Harold Peterson : Arms and Armor in Colonial America

EDIT: I should add that a .69 caliber roundball is 495 grains, and a piece of 00 buck is 60 grains, so adding a few greatly increases the projectile weight. Reproduction muskets are available and great good fun to shoot, but anyone who tries a buck-and-ball load in one should use caution as to how many shot are added, not only for the "strength of their piece" but for the durability of their shoulder!