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Here for a reference on choking smooth bore guns. Short dissertation on jug choking excerpted:
Another interesting thing about chokes is that the difference between the restriction at the muzzle and the section immediately behind it is what determines the choke performance and not the overall barrel diameter.
A method of choking called ‘jug choking’ has long been used to improve shotgun performance - by enlarging the bore for a short length just behind the muzzle. The shot column expands into this area and is then constricted again at the muzzle. Thus, a fixed choke shotgun with fairly open chokes could be converted to a tighter choked option by jug choking. This would not interfere with the original muzzle configuration.
Jug choking is becoming increasingly popular on competition shotguns. While one reason given is improved patterning, the main claimed advantage is reduced felt recoil as the shot column does not ‘hit an obstruction’ in the form of a narrower bore on the way out of the barrel.
Cheers,
Jack