Dry-firing a percussion firearm. . . . Care should be used in dry-firing a percussion firearm. If the hammer is dropped on an empty nipple, the nipple is very likely to upset and develop an oversized rim. Caps may no longer fit properly. (The only exception to this that I know of is the Ruger Old Army, whose hammer is stopped just short of the empty nipple.) A common sign of this is that a cap may not fire the first time the hammer falls. The first fall of the hammer may actually seat the cap, and the cap may fire with the second hammer fall. If this happens, check the nipple to be sure that the cap still seats correctly. Light dressing of the nipple with a file may be in order.