Does a shotgun still have much CURRENT use? Always hear of battle rifles, my uncle told me there were quite a bit of shotguns issued in the Navy due to the close quarters inside the submarines.
Depends entirely on who you're expecting to fight and at what distance. Something like 5.56 will be more effective against an opponent in soft kevlar armor, while shotguns may have trouble penetrating it. A shotgun is going to be very effective at close range though, and will still seriously injure someone wearing even hard armor plates.
I've read that shotguns have a muzzle velocity somewhere in between a handgun and a rifle. So shooting someone with a shotgun loaded with buckshot is a bit like shooting them 9 times at once with a handgun. A shotgun will absolutely shred and unarmored opponent.