RE: Muzzleloading shotgun questions
82 Grains is equal to about 3 Drams. Equal volume load of 80 powder to 80 shot. I would proof test the barrels before I fired it from my shoulder. If you have a bore light take a look down the tubes to see if they are pitted and if they are how badly or deep. If no bore light, a small piece of mirror dropped down the barrel will reflect light. Have a gunsmith proof the barrels or if you want to proof test it load each barrel seperately(one at a time) with a double load (160-160). Tie the gun to something sturdy and place something heavy behind the butt.Take the nipple out and place a fire cracker fuse into the nipple hole to the barrel or if you decide to leave the nipples in and test, rig a line at least 25 feet long to the triggerBEFORE you cap the nipples. If the barrels come out of test OK you can shoulder fire it with standard loads.
#6 shot is fine for Turkey. Set a paper target about 24-30" wide at 40 yards and see how dense the pattern is . Try a load of powder,two cards,shot and an over shot card. The whole thing is how dense the pattern is since you probably won't be shooting 40 yards at a bird. If there are holes in the pattern,try a lighter load. 70-75 Grains. BP shotguns are usually Cylinder Bore so 40 yards is about the best you will want to try a shot. I wouldn't bother with the plastic shot cups.
Rock Lock