http://www.rockislandauction.com/viewitem/aid/65/lid/1197

The Warner carbine was another of the weapons used in small numbers by the Union cavalry during the Civil War. It is a pivoting breechblock action built on a brass frame. These carbines were made in two batches, known as the Greene and Springfield. The first guns were chambered for a proprietary .50 Warner cartridge, which was replaced with .56 Spencer in the later versions (for compatibility with other cavalry arms).

This particular Warner shows some interesting modification to its breechblock, which has been converted to use either rimfire or centerfire ammunition. This was not an uncommon modification for .56 Spencer weapons, as the centerfire type of Spencer ammunition could be reloaded (unlike the rimfire cartridges). With this modification, the firing pin can be switched from rimfire to centerfire position fairly easily.