When the gun arrives, please measure the bore diameter with a caliper. Or have whoever is building the gun do so, and give you the dimension. THEN, and only then, can you order a round ball, and wads of the correct diameter. TOTW has what you need, but then so would Dixie, Log Cabin, and several other suppliers. Get your cleaning jag, swab, bullet puller, and wad puller jags, together with a good range rod. Mine is from Stainless steel, but you can find good ones made of brass, and even aluminum. Just buy one that is long enough for that barrel length, and you will be set.
YOu will want to order flints, wads, cushion, and overpowder, and then over shot cards if you intend to shoot shot loads from it. If you don't have them, get a bag, or make one, and at least one good horn for the powder. You don't need a separate priming horn, as you can use the same powder that you use in the barrel for priming powder. If you don't have one, buy a good adjustable powder measure with a funnel attached, like that sold by Tedd Cash Products.
I am using 70 grains of FFg powder under a .600 PRB. 70 grains of FFg with 1-1 1/8 oz shot, depending on purpose. For rabbits, a ligher load of 55 grains and 3/4 oz. seems more than adequate. That would be a light 28 ga. load.
The benefit of using black powder for muzzle loading shotguns is that it allows you to load up or down depending on hunting conditions, and the game being hunted. You might want to make a shot bag out of leather, and another ball bag of leather, to hold no more than 1 lb. of shot, or 2 dozen balls. YOu can buy an Irish measure from Track and simply sew up a bag from leather, and put the measure on it. It comes with instructions. A ball bag is nice so you don't have balls rolling around at the bottom of your possibles bag. Some have a stopper and some just are tied at the neck, or use a drawstring.
