The second 18.5" barrel for a 3.5" 12ga. autoloader is going to a very spendy option, if you can even find one. You might find a second 24" barrel, but that too will be pricey. I'd focus on selecting a waterfowl gun that fits you, that feels good in hand, and upon mounting. My working waterfowl guns are the Beretta Xtrema II. Beretta autos are unique in that the are both gas and recoil operated. They're soft shooting even with the heaviest loads forced through tight chokes. The Beretta Extema II is "Aqua" coated inside and out. This coating is extremely corrosion resistant. I hunt on and near saltwater and have been amazed at how well these guns have held up. There are lots of great options out there these days. You should be happy with any of them as long as you prioritize fit.
