Shotgun are usually individuals...they kinda like what they like. So you are gonna have to try a few brands of ammo and see what gives the best results. If it were me I'd start with hornady SST, Remminton accu tip and some Winchesters all 2 3/4" the 3" recoil is not worth the few extra fps. You want sabot slugs in a rifled barrels.
Some tips
Shoot slowly and methodically ( new clean target for each brand of Ammo)
Set up a good shooting bench/ platform
Use a good rest (sand bags are great)
Do every test shot/s the same for each brand of ammo.
Fire a minimum of 3 shot group per brand of ammo ( only way to determine repeatable accuracy)
Check your rig after each brand of ammo ( shotgun recoil is hell on rings, bases and scopes)
Shotgun slug testing is hard on a shooter and expensive...good luck, shoot safe and have fun.