Hans Vang describes the modification here:
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The Vang Comp System (VCS), designed by gunsmith Hans J. Vang, is a set of modifications to shotgun barrels. The alterations to the barrel consist of lengthening the forcing cone, back-boring the barrel, and adding compensating ports. With the VCS, accuracy significantly improves, felt recoil is dramatically reduced, and muzzle rise diminishes. By lengthening the forcing cone, the pellets have a smooth flow from the chamber to the bore. This prevents the pellets from bunching up in the barrel and reduces the felt recoil. In addition, the pellets do not become as deformed and keep a more consistent shape.
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Probably one of the most important and most obvious parts of the VCS are the compensating ports. Some of the gases, created by the burnt powder exiting the muzzle, vent upward through the ports. This redirected gas flow helps prevent the muzzle from rising and allows for both increased accuracy and a more rapid recovery time between shots. Also, the ports reduce muzzle flash, which in turn helps to retain night vision and keep the shooters location less conspicuous in low light conditions.
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And patterns are shown and defined here:
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In my opinion, Federal has achieved with ammunition design, the kind of pattern that used to be available only through the barrel modifications Hans Vang developed.