for a reference on choking smooth bore guns. Short dissertation on jug choking
The 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.