I think a couple things should be mentioned.
1) Shotgun send a lot of metal balls at an enemy that spread out, so It wouldn't really be a good long-range weapon.
2) If you do get a good shot where it could hit someone, It could most likely hit more than one person, which would be a good thing for a sniper, but overpowered in some cases, such as this.
So if this is made, instead of the traditional shotgun shells, use slugs. That way you can at least have one specific target instead pf hitting multiple people and killing them all withing the radius. And so it can go a longer range then the traditional one.