Cast off is associated more with shotguns than with rifles, but a good rifle stockmaker can provide you with cast off, ( or cast on) to help you fit your stock. Cast off is the amount to which the centerline of the stock as you look down at the gun from above, is made off center. For a Right handed shooter, the cast off goes to the Right, or to 3 o'clock. The purpose is to allow your eye to gain an alignment with the center of the barrel. With a shotgun, your eye is the " rear " sight of the gun, so the position of your face on the comb of the stock is critical to centering your pattern. By making the stock so that the butt of the stock is "cast" away from your face, the stock makes it easier to get your eye directly in line with the barrel, while keeping your head straight. If you don't have to lean your head over to center the barrel, you are much less likely to lift the head up before the shot is fired, either to avoid being slapped with recoil in the cheekbones, or to relieve the stress on your neck muscles. How much cast off you need depends on your facial shape, the length of your neck, etc.
"Cast-on" is simply a stockmakers term to describe doing the same thing for a Left Handed shooter. It allowed the stock fitter, to communicate accurately with the stock maker, often two different people in these high end gunshops. You can still experience the same at any of the English Best Gun makers shops, and here in America at Orvis, and a few shops on the West Coast. Once you have a gun fitted to you, and learn to shoot it properly, you will want all your guns modified to thos demensions.
