Guns which will work well in low temps rain sleet etc are many, but add wind blown sand or mud into the equasion it needs a special gun to keep on spitting them out reliable.
The side by sides and O/us are of course reliable in that second shots are at least generaly guaranteed, but with just a little mud or debris in the action frame hung up in the bite its a no go wont shut and function . So unfussy as they seem on the face of it they can and will give issues.
Now the drop of the barrels to reload is good in that you can check bore is not plugged by an acidental touch down in your shooting possition or hide. so checking is arguably more nessasary, OUs have a big gape need a fair old drop to get the bottom barrel, so again loading can be a pain holding an OU up in the air as you shut it too, whilst keeping your muzzles clear of hide net mud dogs hide poles is a task all in its self.
And to add to this sat low in a possition putting shells in the barrels tend to be angles slightly up hill shutting the breach involves lifting the breach but stock hulls do tend to want to fall out land in the mud water boat grass whatever and saltty mud eats chambers if your not careful about your gun hygiene.
I feel the SxS is the best in the double guns for fowling they have less gape for loading in tight areas are generally lighter to equivalent Ous. Now finding multichoked SxS guns in 3.5 inch is tougher than say Ous semi autos or pumps, but plenty of 3 inch and 2 3/4 inch SxSs around still able to be capable performers on waterfowl.
So in general i believe INLINE action designs rule, Pumps autos these have disadvantages checking for the previously mentioned mud plugged muzle is not as easy but loading is a synch muzle always faces up hill easy to load and unload, in fact pumps and semi autos are way eassier to unload in a seated rock pile on a shoreling or that mud hole on the green shore . pumps have the advantage of manual functioning where in a filthy contaminated gun a little bit more mussle power saves the day, the same with wind blown sand or snow blown in the port and froze up.
But semi autos do have built in lower felt recoil for big loads which might be of some value to some waterfowlers but not all.
So now makes and models, PUMPS. Model 12s win 1200 mosberg 500 *35 Moss/ mavrick 88 rem 870 all work ideal along with nova and supernovas, but personaly i would go portless with BPS or Ithaca Mod 37 these bottom eject less ingress of snow water debris and in pumps i think they are the way to go. My choice would be the BPS hands down. Not the slickest action but certainly tough and dependable at practical price point.
Semi autos, Now again many will pass muster and function up to a point in waterfowling, Berttas extrema A 400s all the 300 series 301 302 303 390s and the urekas outlanders etc all great guns, but i have seen them get sticky when the snow gets in them snow sorts the men from the boys out in a waterfowl gun, build quality is all well and good, but a helpfull amount of designed in slackness in the working parts is a destinct advantage in waterfowling semi autos. Here the Bereta range good as they are get to see jams when the sittuations start to get cold and bad, Next up and again my opinion Browning golds maxus Win sx2 sx3 sx4s These if lubricated and clean function great, but hang ups can happen , again they hate\ snow rain too and although a Browning fan myself i have to admit they do give issues probably more than expected in poor conditions.
Benelli Autos these are often claimed to be the best waterfowl guns, well made inertia opperated super tough, but in use they too give issues when it gets cold, and the benelli semi auto ranges generaly fussy lubrication requirements can start to bring about issues in the cold and for me at least inertia or not they get side lined because of this just like berettas brownings etc We must touch on the new A5 its inertia and it is a performer but as the benellis the short recoil inertia guns can give issues, but i would pick a new A5 over an SBE for reliability in poor conditions but i would pick a vinchi over all of those short inertia guns mentioned. .
Inertia as its merits but i believe we must travel back in time to a bygone age to get true inertia toughness and reliability, The old A 5 the long recoil king this gun just never failed to impress its tough reliable keeps on chunking away in blizards whatever and apart from the wooden stock on most examples are pretty much the perfect waterfowl semi auto for reliability certainly the longest lived and best made ever.
Big downside in the OLD a 5 is the need for turn screws to field strip these guns this aspect steals them of the crown in my eyes, and in long recoil semi autos i chose to own three in the Franchi 48 A magnum 3 inch these are technicaly inferior to old A5s but in magnum have steel action frames and pins to field strip and to be honest are quite close to perfect waterfowl guns let down only by woodwork and unavilably aftermarket synthetic furniture, some demand in an ultimate waterfowler.
So as we close we have missed out OH so many guns Stogers Hatsans older winchesters mossbergs 9200s 935s and others 935s are incredible waterfowl guns but low powered ammo typicaly used in decoying ducks can give issues in opperation on some 935s but they are indeed exellent and the 10 bore barrel may or may not be of interest to you.
So the winner for me in any case is the Baikal MP153 its cheap nasty crude ugly call it what you want but it keeps on going year in year out i have had one 15 years its yet to ever jam and its seen action in snow filth rain sand you name it its flawless functional perfection cycles 7/8th to 2 1/4 lead loads on one gas setting the highly toleration of debris action design and build mean dirt normally encountered change nothing in function it just spits them out regardless.
We all know there are better guns out there but the MP153 is like the AK47 its not a AR15 its not a presice tool its a functioning brute of a gun built to survive in the russian environment maintained with the minimum of tools and knowledge super simple super tough and Cheap. For me they are where 12ga waterfowl guns need to be to function in the cold harsh environments we chose to hunt in.