RIP on AR Big bore forum is doing 835 Mossy and 1400gr Darwin slugs.
A 14.5 pound Ulti-Mag. The hollow part of the butt is filled with about 5.5 lbs of lead shot and epoxy. Has 4X Sightron in QRW rings. Do 3.5" plastic hulls with the Darwin in it first. It balances on the trigger. Loading the magazine with 4 and one up the spout will add about 1.25 pounds if the 5 rounds are "Darwins," and move the balance point to the front of the triggerguard.
It is heavier rifled 24" chromoly alloy barrel.
Brett in MN has Mossy 835, only with smoothbore barrel and gets
1oz finned plastic tail Ballistic Products slug to 2300 fps,
using shotgun powder.