All shotguns have the same gun ability if you have at least a +1 bonus with it:
You have advantage on attack rolls against creatures with medium or heavy armor.
Shotguns have the following calibers available to them:
Average & Heavy
KSG : 50 Gold
1d8 Piercing + Str/Dex, 20ft Range
(2 Shots Until Reload)
Mossberg 590 : 85 Gold
1d8 Piercing + Str/Dex, 30ft Range
(4 Shots Until Reload)
UTS-15 : 105 Gold
1d10 Piercing + Str/Dex, 20ft Range
(4 Shots Until Reload)