Re: copper coated steel
Copper coated steel BB's work fine in 12 gauge shells if you have a newer shotgun to shoot them in. I don't know what you are hunting, but about the only time you would want to use "BB's" is to hunt geese. BB's will do too much damage to the meat of smaller birds, and that is if you manage to hit one with a BB pattern. If your shotgun was made prior to the early 1990's, and does not have interchangable choke tubes, then you cannot use steel shot of any size or coating. Your only option is a non toxic load using "Bizmuth".
If your shotgun has interchangable choke tubes, you can buy hardened choke tubes that are specifically made for shooting steel shot.
Copper and zinc coatings on steel shot are not thick enough to act as a sufficient buffer in older model shotgun barrels. What will happen is that the end of your barrel willl become "pinged" up, which thins the metal making it split. In an extreme case they could bulge or split the barrel within a few shots.