The Vulcan 3 700mm in .30 cal is not an "excellent slug shooter" because AT (airgun technology) used oversized barrels .3064 from a company other than advertised CZ.
The gun shoots pellets fairly well and .300 slugs that have a base similar to a pellet skirt to seal the gap between slug and oversize grooves.
The adjustable plenum thing is more of a gimmick since the small side is plenty for .22 and the large side is only needed for high power .30 cal. and causes a prominent ping.
I still like the V3 so I'll rebarrel mine to the industry standard .294/.300 bore and groove size so the gun can actually be as good as advertised.