1.  Field of the Invention
The present invention relates to a slug assembly for a shotgun shotshell. 
2.  Description of the Related Art
A slug load of a shotgun for animal hunting or target shooting usually comprises a case with a primer, a powder, an over-powder wad, a wad, and a slug.  The powder, the over-powder wad, the wad, and the slug are inserted in the case in this order and the slug is held in the case by a crimp provided on the front open end of the case.  This slug load has the following drawbacks. 
(1) The grouping which represents a hit accuracy is worse than that of bullets of a rifle-gun.  In addition, occasionally some of the slug mark(s) are deviated widely from the line of sight. 
(2) Since the slug is usually made of lead and is naked, a large amount of lead becomes stuck to the inner surface of the barrel bore of the shotgun, and much time and labor is needed to remove the coating of lead.  If slugs are fired from a barrel having a coating of lead, the grouping of the shotshells will be very bad. 
(3) Since the slug load has a heavy slug and is fired at a high muzzle velocity, the user receives a heavy recoil.  Thus, apart from animal hunting, in which the rate of fire is relatively small, a user tends to become easily fatigued, resulting in a decrease in the hit accuracy, particularly when practice shooting during or a shooting match at a stationary or moving target.  In addition, the heavy recoil causes the users, particularly beginners to become gun-shy. 
(4) Even though of the same gauge, shotguns have slightly different barrel bores, different barrel lengths, etc.,  dimensions and different choke shapes, depending on the manufacturer.  To obtain a high score in a shooting match, the users must carefully select slugs which match the shotgun used.  If well-matched slugs are not available on the market, the user must try to make good slugs, by themselves. 
Regarding the problem of bad grouping, the outer diameter of the slug is generally smaller than the inner diameter of the barrel bore, and accordingly, when the slug is fired from the barrel, the slug may be inclined with respect to the axis of the barrel, or the slug may wobble when passing through the barrel bore.  Furthermore, the rear portion of the slug is usually hollowed out so that the center of gravity of the slug is located toward the front of the slug, taken along the length thereof to provide a heavy head, and has a small tensile strength because of the lead or lead alloy of which the slug is made.  Therefore, during the extremely rapid acceleration of the slug when the gun is fired, the slug may buckle at the hollowed out portion thereof, resulting in an asymmetrical deformation with respect to the axis of the slug.  This worsens the grouping. 
In addition to the foregoing, if the axis of the slug is inclined from the flight axis during the flight of the slug after leaving the muzzle of the gun, no effective stabilizing means is provided for restoring the posture of the slug.