Its all in how much you enjoy patterning...There are countless options setting up your turkey rig...An acceptable hunting pattern can be what the shooter deems all he or she needs for their range preference.. Lead had killed turkeys for years and will continue until non toxic shot regulations force it out of the game..for the most efficient patterning hevi shot, tungsten are above and beyond the capabilities of lead...With the Nitro, Hevi 13 or Fed HW pellets the performance can be made outstanding once you've matched a choke with a polished barrel and/or choke that compliments the load...For bare bones affordable performance with that 500 mossy and Hevi 13 6's the carlsons .660 as mentioned by another post is a great start..Shoot the Hevi13 3" 2oz 6's or 7's. Nitrocompany ammo would be your next step but the mentioned loads will be great..

Good luck on your setup!