Last spring I shot a Gobbler at 44 laser verified yards with the White Thunder.
The load of 1-7/8 oz of magnum #5 hard lead shot inside a Ballistics Products Turkey Ranger Shotcup (4 slits cut down to the bottom of second internal ring) over 90 grains of 777 FFG, topped off with 2 of my homemade styrofoam over shot wads! Why 2 overshot wads, you might ask? By using 2 overshot wads, the second one acts like a jam nut on the first, if you have lost your load prematurely , you will understand! The styrofoam is light, seals out moisture, and doesn't swell or fail if wet from rain.
That exact load is good out to 50 yards in my White Thunder with .665 Hastings II extended choke tube!
What choke tube have you got? What is the constriction? What threads does it have Rem-choke, Win-choke, Hastings II or other?
ballisticproducts.com has several hunting wads, and even free loading manuals in PDF format like for the Turkey Ranger Wad that I use. They also have Heavy Shot wads, steel shot, bismouth etc,.