When I was a kid, my cousin married a big ol' backwoodsy logger from the Clearwater country in Idaho. Leroy hunted with an 8-gauge double,for which he had brass shells he reloaded. (Leroy was a real "use enough gun" man; he left my cousin alone one logging season in a cabin on the Middle Fork with two babies; she had to shoot a black bear in her kitchen with Leroy's .405 Winchester--and her not 100 pounds, dripping wet!) My other cousin packed a lever-action 10-gauge Winchester when they hunted together. Those days are 50 years gone, but I remember the bags of ducks and quail--yes, even quail--that they brought home sometimes. For the past fifteen years or so I've hunted all my ducks with a reproduction Pedersoli 10-gauge muzzleloader, and it's been great fun; I probably don't kill as many as I once did with modern shells, but they taste better. I won't publish my loads online(they're a bit hefty) but I will say I can get about the same results with steel shot as a modern 2-3/4" 12-gauge; i.e. 1-1/8 oz. of steel at 1275 ft/sec. Punt guns are still legal in the British Isles; look for an article in Gray's Sporting Journal about five years ago. Every major gun show'll have at least one 10-gauge, 3-1/2" double, often for less than $500.00, so you're not out of luck; keep looking!
Mind yer topknot! windy