Ehhh, shotguns don't make very good sniper guns, and I'll give you three reasons for why that is:
#1: Shotguns don't have as far a range as sniper rifles do. #2: Shotguns are loud, while sniper rifles are silent. #3: Shotguns fire pellets, or "Buck Shots" that scatter over a wide range, while sniper rifles fire bullets that go straight over a long range, and are much more accurate than "Buck Shots".
In short, shotguns are best used in close-quarter gunfights while sniper rifles are....what the.....HOLY SHIT! SHE'S NOT WEARING ANY UNDERWEAR!!!
#2: Sniper rifles are extremely loud unless you attach a silencer. Even with one they aren't silent. That is a myth.
#3: Shotguns can fire a wide range of ammunition types beyond just buckshot. For example, slugs are not pellets but a single large "bullet". Furthermore, long range rifle projectiles do not "go straight", they arc based on force, gravity, wind, and many other external affects.
All that being said, the words "Shotgun" and "Sniper" should never be used together.