I have an Undertaker on my Rem 870 and it likes 3" Winchester High Velocity 13/4 ounce #5 best, with the same load in #6 a close second. I have tried all of the 3" Rem., Federal and Winchester loads in #5 and #6 that I could find, and these two seem to stand out. I think that my pattern testing cost me almost as much as the gun.
Curiously, the factory extra full choke liked #5 best also, but in Federal 2 oz. Actually, the patterns were almost as good as the Undertaker' s.