I know this post is old but: I own an 835 & the Mossy slug barrel w/ cantilever...here is what i've learned: I love the cantilever, i put a bushnell banner scope on it, great optic. I love that i can switch from deer to bird gun w/o messing w/ the sight. I also have truglo florescent sight on the smooth barrel. Also got a Boyd's thumbhole stock & the Knoxx recoil reducer... I love em both over the normal stock.
i started using Hornaday HST (the best ammo i could afford at the time) and I was shooting minute of barn-door at 100yd. I tried Remington, Winchester, Federal, rifled, foster, smooth, cheap expensive etc ad-nauseum & was not getting any better. What i discovered was that the rifled barrel has a 1:36" twist...which uses loads shooting less than 1500fps, all the ammo was shooting over that, some waaaaay over, like 2000fps (HST's). I found Federal non-rifled slugs locally shooting 1450-ish getting a 5-shot group w/in a foot @ 50yd. I figured I would roll my own & see if i could get any better, I got a Lee 1oz slug mold & found very few recipies, the last time i went out I uased 40gr H38 and was shooting minute of yardstick...then i read that was the max load...not the starting. So I still am looking for good slug info using the Lee slugs...I cant see shelling out $90 or better for a Lyman mold...unless I really cant get anywhere w/ the lee slugs. Im befuddled that Lee doesnt have any GOOD loads for each of their molds (the way Lyman does)...you get what you pay for i guess. I will post any future discoveries. Good luck & post back what you found.