[Ron Wood]

Adam,
That artillery barrel that you bought was correct (except for the fact that it may have protruded too far into the breech and would have needed to be faced back and headspaced), and with it you have learned something about artilleries. The sight is supposed to move to the right as the sight is elevated to compensate for the driftof the bullet at longer ranges due to spin imparted by the rifling. Also, if you look at some close up pictures of artillery Lugers, you will see that the receiver is notched on the top forward edge to clear they rear sight when it is in the down position.