P-man, best thing is to get several different brands of slugs and go test them at the range. That will be the best way to determine accuracy. Firearms can be a bit individualistic wrt to ammo preferences.
There are a lot of things can influence accuracy. A lot of times the gun/barrel gets the blame when how the scope is mounted is the problem. I've never checked the twist on Rem barrels but 1 in 35/36" is the norm for Winchester slug barrels as well as Mossberg.