Just learned some stuff about the shotguns.

-No headshot multiplier
This means if you shoot your opponent in the head you only get 1x damage instead of the typical 2x. So if your opponent is wearing the armor in the "defensive" setup then you better be very close to get a wicking kill. The armor reduces body and arm damages by 33%.
-Flechette ammo negates the "defensive" body armor's 33% damage reduction so you get the full damage of the shot.