Found some nice pix of the Mateba auto revolver.  I did not know that there was a .454 Casull version.  Alucard would approve, I'm sure.

UPDATE:  For the benefit of readers who aren't particularly familiar with guns, or with this gun anyway.  There must be at least one of you.

Most revolvers are not "automatic" in any way.  With a single-action revolver, the cylinder is rotated to a fresh chamber when the hammer is pulled back, then the trigger is pulled to fire it.  With a double-action revolver, pulling the trigger first rotates the cylinder and then fires it.  The Mateba auto-revolver uses the recoil of firing to rotate the cylinder to a fresh chamber.  The first shot must be double-action, but the recoil action also cocks the hammer so following shots are single-action.

Another thing that sets it apart from traditional revolvers is that it fires from the bottom chamber of the cylinder rather than the top (check the pictures to see what I'm talking about).  This must give it a very different kind of recoil from traditional revolvers.

And needless to say, I would really like to get my hands on one.