Do you mean "jacket" or "case". I assume you're referring to "bi-metal" ammo—the copper washed, steel "jacket" stuff is fine in a chrome lined barrel/chamber. Except for "cleaning issues" I do not believe it will cause any additional wear on a regular molly barrel either—if cleaned right. If you're referring to steel "case" I know that some say it's hard on extractors, but I haven't seen it. I do see a lot more smoke/carbon down the neck/shoulder of a fired steel case (than brass cases) and subsequently in the chamber... again requiring more cleaning. Bi-metal— the steel jacket under the copper wash is very soft, I've experimented by cutting them up with pliers annealing them etc. Given that the steel part is not self lubricating in the same way a regular copper jacket is, it might be a good idea to allow the copper to build up in the bore... just a thought.