start off at 3 1/4 with 90 gr. powder and work up from there.see how you gun does with each load.
shoot at least 3 to 4 loads of each change.
I have a CVA 12 ga. double.
I shoot 3 1/2 of #6 shot with 125 gr. of powder.that works real well for my gun.(maybe I should say my wife's gun) I got it for her about 20 years or so ago.now I use it for turkey sometimes.
I started off with #6 shot mixed with BB's. I went back to just plain copper plated #6 shot. that works best.with that load I am comfortable to 30 yards with a good open shot.I try to get them in even closer.thats the name of the game ; close , close and even closer.
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it's fun working up the right load and shot size.