If those modern shotgun wads are a "Tight fit", either your barrels are choked, or its likely closer to a 13 ga. rather than 12 ga. bore. Modern shotgun shells use 13 gauge plastic wads in them, because of the added thickness of the plastic walls of the shell.
I would not use plastic wads in any gun using Black Powder as a propellant. You rub off plastic against the sides of the bore, causing more tight spots, where both more plastic, and soot gather, to make seating successive loads more and more difficult. It then requires modern gun solvents to remove the plastic, where soap and water will remove just the soot.
Unless your shotgun is marked "Safe to use Steel Shot", do NOT USE STEEL SHOT IN THE GUN! The steel used is not metallurgically designed to withstand the hard abrasion of steel shot! Even using the correct shot cups/wads, you risk damaging the barrels.
Use Black Powder muzzleloading shotguns with black powder, at black powder velocities, and hunt closer, or learn to wait for birds to come closer to you before you shoot. There are now much better non-toxic shot types available to use in BP guns, than steel shot ever was, or can be. Look for bismuth, polymer-iron, and the newest XTL shot sold by Ballistics Products, which has a atomic weight very close to that of lead, and is soft enough to be used in older steel barrels. It apparently also patterns like lead shot, and transfers energy to the birds like lead does. The few members on this forum who have tried it speak highly of the product.
