I don't hunt deer with 00 but I do hunt coyotes with 3" #4 copper plated Buck and have found Federals give me the best pattern. I would suggest that you buy some different brands and pattern each brand to find out which is best in your Mossy you might be surprised, try different chokes also extra full turkey works well for my needs. Do you have to use Buckshot? Slugs will give you more range but because of an oversized barrel your Mossy will need a slug barrel or you could buy another Mossy just for slugs.
Oats that have already gone through the horse are always cheaper than oats that haven't !