The .665 is where'd I place my bet. I had a 500 Mossberg that was unreal with a .665 and basic 3" turkey loads. The tighter constriction is going to limit you to the smaller sized shot... which is not a good thing IMO.

I'll betcha dollars to donuts that the .665 with #6 Hevishot will be a real thumper, (be my #1 choice for that rig) or if you want lead only, try that same .665 with Winchester Supreme 3" High Velocity #5's. You'll have a winner either way. Yessiree.