While it can successfully be argued that bullseye shooting can teach a lot about marksmanship, it is also a fact that marksmanship is but one-third of the Combat Triad (the other two sides being Manipulation/Tactics and Mindset).
Shotguns have been used as combat weapons since the matchlock. The fighting shotgun reached a pinnacle in the "trench gun" of the early 20th century—an 18-inch barreled Model '97 or Model 12 with a bayonet, spraying buckshot into groups of enemy soldiers at close range. United States soldiers used shotguns in World War II, Korea, Vietnam and they are still being used in the Middle East today.