Hey there fire-n-b,
You most certainly can hunt deer with roundball out of your 12-gauge. You really need to know the size of your bore, because that can vary between manufacturers. But 12-gauge, as you probably know, runs around 75 caliber.
You might try .680 balls as mentioned above, or you can also try .715 or even .735 balls. You don't want to get too tight in a smoothbore, because there's no rifling to take up extra the space of a patch.
In my own experience, I have always had good luck with an over powder wad, a 1/2" fiber cushion wad, roundball (patched or unpatched, just don't patch it too tight), and then I always use an overshot card, but if you're using a patch that may not be necessary.
The theory behind the fiber cushion wad is that the wad will give a little, allowing the ball to sink into it a bit. This helps to keep the ball centered as it moves up the bore. It helps with accuracy - I know I've certainly had good luck with this type of load.
That big ball will put the whoops to a deer. :grin:
Good luck with it, whatever you decide. Please do keep us posted.
:hatsoff:
Spot