I hunt mainly with a Remington 1187 20ga. I shoot with a modified choke. I mainly shoot 4 and 2 shot. I seem to have a bit better luck with the #4 Remington Hypersonic Steel than anything else.
I would recommend getting a couple of chokes and several different types of ammo and go pattern your gun. What works for me and my gun might be totally different than what you feel comfortable with.
I shoot 3" in my 1187 but my Mossberg 500C wouldn't cycle it as well as I had hoped.
As far as availability of ammo, it is true that there is not as much of a variety, but you can get most of what you want. You may just have to go online. I went to Wisconsin lasgt summer and found a whole new world of ammo that we never see this far south, so do some research and you might be surprised in what you can find.
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U.S. Army Military Police (ABN) 1998-2003