I dunno how stout 311's are in general, but I shot a 20-gauge loose during the 80's.

I love the old American doubles myself, and have owned a bunch, including Foxes, Elsies and Ithacas--and have a very nice Ithaca Flues 12 right now. But the early ones (like the 1911 Fox I had for a while) often don't have modern buttstock dimensions. The 12's also tend to be on the heavy side compared to European guns.

Another I should probably mention as a possibility is the original Beretta Silverhawk. They were basic but very good boxlocks, and you can still find them for under $1000.

“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.”
John Steinbeck