Hello and welcome to the site!

A look at your other two posts shows you were there to help one of our fellow members while he was in the hospital -- big thanks to you for the sacrifices you make sticking to your vocation. The emergency services are tough and at times thankless jobs.
Keep up the good work.

On big shotguns, there's been everything from .22 rimfire on up to light artillery sized bores.

When one speaks of shotguns in terms of 'gauge' or 'bore' (synonymous), the number (as in 16 for 16gauge/16ga/16bore/16b) refers to the size of a ball of lead, one sixteenth of a pound (an ounce) 12ga = 1/12 pound, 10ga = 1/10 pound etc, whose diameter would be the bore diameter of the gun or rifle.

The large 'bore guns' were for the most part as 'market guns' by commercial bird hunters who'd take large numbers of birds per bag to sell to the local market.
You'll also see 'bore rifles' of similar relative size.
The larger bore rifles were the big dangerous game rifles of choice before the high velocity 'express rifles' came into vogue in the late 19th century.

Tour around this site for a while, look through the Big Bore rifles section and the Paradox and Bore Rifles section (seen via the Main Menu), also use the search tool to run some searches specific to your quest.
There's acres and acres of information here, much of it first hand experience of our members' own guns and rifles, handling them - loading for them - and hunting with them.

Also you'll see that there are modern builders putting out modern and vintage-period examples of these large guns and rifles.
One particular member, SZIHN, builds absolutely beautiful replicas of 19th century high grade sporting arms - do a search for him and you'll see some very nice pieces. The prices are definitely reasonable -- much more than a Mossberg, but definitely worth the price. He's even made some very ornate and beautiful Blunderbusses!!

I hope you enjoy the time you spend here.
Also, I hope you find your way into the right large bore shotgun or bore rifle to suit your interest.

--Tinker