Nasty, you are a fabulous resource! That Dickenson shotgun sure looks like a winner, but the fine print says that it comes with a cylinder bore, no choke tube ability. Too bad. I'd rather have choke tube option rather than the muzzle brake. If we could change to a rifled choke tube, that short shotgun looks ideal. I may phone them and ask.
Ditto re 3 inch rather than 3 ½, and I have some 2 ¾ cheap ammo on hand. For deer I'll take the 12 pellets of 00 buck in a 3 inch shell over the nine pellets in a 2 3/4 shell. Ditto for higher velocity with 3 inch slugs. Not nearly as many options available in 3 1/2 inch.
I got such a deal on the Mossberg that I bought it, and so far have proven that a good deal for the sake of a good deal is not.

By my first coffee this morning I decided that I may change guns rather than pour more money and accessories into what I've got. A Savage bolt 20 gauge with rifled barrel and scope is another option, accurate to at least 150 yards.
The main good takeaways from what I've learned so far is that rifled choke tubes work, in this case anyway, and that with a cylinder choke in my gun, Federal Flite Control Wad buckshot holds a tight pattern to at least 40 yards.