Fordtech - don't assume that muzzleloader is a wall hangar. Two years ago I had our family heirloom 14ga muzzle loading side-by-side restored to shooting condition. The gent who did the work is a multi-time national muzzle loading skeet champion. He was some ticked off when he had to admit that in his test firing he only broke 24 of 25 birds with it. This shotgun came over from England with our family in 1853 and has been responsible for putting grouse, rabbit and venison on the family table for at least a century of regular service. I am extremely proud to have it, especially now that it is very much in shootable condition.
If you ever decide you don't want that Model 37, I would love to have one in 20ga for conversion to a lightweight version of Ithica's "Deerslayer". The factory "Deerslayer" appears to only have been made in 12ga. I much prefer the lighter recoil of the 20ga, so I'm keeping my eyes peeled for a Model 37 or the more recently-produced Browning BSS. (The original design was by that most prolific firearm designer, John Moses Browning.)
As for the OP's question re the Mossberg 500 and a rifled slug barrel, I have one of those and it's my go-to gun for home defense. That rifled slug barrel produces a man-sized pattern with bird shot at 7 yards, making it the ultimate "point and shoot" device for very short range. Not that I would ever use it for that purpose, but it sure is nice to know about this capability...
I have never found the Model 500 rifled barrel produced noteworthy results with any of a broad selection of expensive sabot slugs. All of our shots here are short shots so I do my group testing at 50 yards. It will reliably cloverleaf three inexpensive Remington "Slugger" non-sabot slugs at 50 yards. As mentioned by a previous poster, the felt recoil from this gun is prodigious, thanks to just about the worst stock design I have ever seen. I've drastically re-worked my stock so I can actually stand to shoot the darned thing. It still punishes me, but at least I can get off one well-aimed shot and have a reasonable chance of a decent follow-up shot if necessary.