I still think the best pure investment strategy for shotguns is to plan a trip overseas, making prior arrangements at the various auction houses, then buy every Winchester Model 21 you can afford. Bring them back here and sell them.
With all due respect, that is a lot easier said than done. I never saw an American gun in Europe, save one Remington pump deer rifle in .270, sitting new in a dealers rack, where it had been for years. You don't just take possession of a firearm in most of Europe, and you don't just come home with it, after you have bought it. Shipping arrangments have to be made, customs duty, exise and state taxes have to be paid, and then you have to sell it. Model 21s, in 12 gauge, are not exactly flying off the shelves here in the US these days, to be fair, most 12s have gotten to be a hard sell.
I wouldn't try it. I wouldn't have tried it when I was in the business, either.
Best,
Ted
