From the Nitro website:

http://www.nitrocompany.com/

Hevishot® Turkey Load With A .670 2" Extended Rhino Choke
or a Primos Jelly Head .670 Choke
H51013 #4x5x7 or #2x5x7 Hevishot® 230-270 pellets

I recommend you follow this suggestion. Nitro has conducted all the R&D necessary,and has determined that this is the best combo for this gun. That being said, every gun is different, but I have followed Nitro's recommendation on three different guns, and have always has exceptional patterns.