Heaviest Load is not the question you should be asking. That question indicates you are thinking in terms of Smokeless powder loads in modern guns.
This is a Black Powder gun. Its an OLD BP gun. It has NO CHOKE. BP does not produce the velocities that you can get from Smokeless powders. Because of the lack of choke, this is basically a 25 yard gun, and you don't need lots of velocity at the muzzle to kill a turkey at that short range.
What you need instead is the ability to call the bird in close enough to you. You will experience the frustration of seeing birds come inside 50 yards, where, if you had a modern shotgun, with full choke, or " Extra Full Choke" you could kill the bird, but because you only have your Civil War Vintage BP shotgun, you don't dare take the shot, and perhaps cripple and lose a fine animal.
You have a 17.5 gauge shotgun, half way between an 18 gauge and a 17 gauge. You should be able to order wads from Circle Fly in 17 gauge; if not, use 16 gauge wads. They will be tight, but then you won't be shooting a lot either. Stick to LIGHT 16 gauge loads, to protect the gun. Light loads tend to have MVs under 1100 fps, and that will give you a tighter pattern than if you go over that velocity. At short range, use #6 shot. Test fire it to determine POA vs. POI on paper, and stick with loads using 60-75 grains of FFg powder, and 1 oz. of shot. There are 225, .11" diameter pellets in an ounce of #6 shot, and at 25 yards, you should have enough hits in the neck and head to kill a turkey.
Put out both hen and jake decoys to bring them in close, as in Spring seasons, the Older Toms are as interested in fighting off the young jakes, as they are loving the hens. Yelp calls that immitate hens bring the Toms to interest in the girls, but a " gobble " call that imitates another tom, or " jake " will bring them in running and spoiling for a fight. At least, that is the theory. I did see it happen once.
