The ARP shotgun is a 3 round burst shotgun that you can mostly find in the Facility and Archives mission. It is very effective in close quarters, and ADS mode is rather unnecessary and actually reduces accuracy.
Trivia[]
The ARP shotgun from a different point of view
It is interesting to note that this shotgun shoots slugs as opposed to buck shot.
It is a real life Russian "Vepr" 12 semi-auto shotgun with both a very incorrect 50 round magazine and 3 round burst.