Did that include the threading?
No theading for chokes.
The whole barrel becomes a choke.
http://www.tacord.com/vangcomp.htm
The Vang Comp System: Modifications to Shotgun Barrels
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.
Higher Rate of Accuracy
With less deformation and a less turbulent flow, the pellets achieve a higher rate of accuracy. Back-boring the barrel is accomplished by boring the barrel from the chamber towards the muzzle creating a choke where there was none. Back-boring is a key portion of the VCS modification and aids in both increasing accuracy and reducing felt recoil.
Compensating Ports
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.
Reduce Felt Recoil By About 15%
In combination, the lengthened forcing cone, the back-boring, and the compensating ports make up the Vang Comp System. The VCS has been estimated to reduce felt recoil by about 15%. The increase in accuracy is dramatic. For example, patterns using 9 pellet 00 buck are as tight as 2 3/4" at 15 yards and 10 to 12" at 25 yards. Also of note, all modifications, except the compensating ports, are internal.
Maximize Performance
No extra parts are added to the barrel. Thus, the VCS barrel requires no maintenance other than normal cleaning. Also, other types of specialty ammunition, such as slugs, can be used in the Vang Comp System. The VCS, developed to maximize performance of the tactical shotgun for Law Enforcement and Military applications, is covered by three U.S. Patents.
