NOTE: Add 30 percent for 20-gauge Magnum. Factory ventilated ribs command a premium of about $1,800 on 12-gauge; $2,500 on 16- and 20-gauge. Model 21s with factory furnished extra barrels will bring an additional premium of about $2,500. Refinished and restored Model 21s are in a somewhat unique category of American-made collectible shotguns. A gun that has been professionally refinished by a master craftsman will bring approximately 90 percent value of factory original guns.