20 vs 12 / single vs double
I'd start out by saying that I think that you should always use the right tool for the job. I understand you may want to try to accomplish many things with that hammer, but in the end, its only a hammer, and it was made to hit nails, not screw things in or weld things together...you should get the tools to do all the jobs needed.
That being said - I've been hunting doves and upland game for years, and until just a couple of years ago thought that the 12ga was the answer to all my wingshooting needs. But i've come to understand the value of having a lite-handling, quick and easy to carry 20ga for the doves, quail and smaller birds. I've found that my shoulder usually appreciates the gesture as well, with less visible bruising and such.
I keep several shotguns for the different types of hunting that I do, Ducks get to see my dirty 12ga FabArms Auto in black (or mostly mud and rust color). Pheasants deserve a more stylish gun, so they get to see my Rizzini's over/unders - usually in 20ga, but depending on conditions, they may see my 12ga's as well.
As for The gentle doves, (internationally known as the symbol of peace), depending on the conditions, and the amount of shooting, and the daily bag limits...and just how sporting i'm feeling, will usually see a Rizzini 20ga - nice and light, with the added benefit (answer to part 2 of your questions) of having an additional barrel which will also have a different choke.
The significance of the additional choke is that i've hardly ever seen 2 birds present the exact same shot at the same time - they are usually going to be moving (unless it's a really slow day in a duck blind), and so to be able to shoot 2 separate patterns at your different target presentations can be very helpful. Example - close target moving away - shoot your more open choke first, followed by your second tube which will provide a tighter pattern for the further hit (unless you're like me and don't need any follow up shots
) of course the scenario can be reversed as well...
So the only time i use a single barrel is if I need the additional round (3rd) in my autos that won't seem to squeeze into my o/u's - Ducks for example usually get all three shots plus an empty click or 2 from me, but that's because I hunt in California, and we just don't get to see enough of them.
If you really only can have 1 gun, then make it a 12 ga Remington 870 pump, and get an extra slug barrel for larger game hunting uses.
Hope that's useful.
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