(the Greener appears to be ever so slightly tapered and the fired shell from my Belgian gun hangs a bit before fully chambering)
I do not recall ever seeing a shotgun chamber which was not slightly tapered. normal taper is about .005" per inch or .0125" for a 2" chamber. A sizing ring can of course only size a single diameter. This is generally adequate for paper or plastic hulls. For serious work with brass shells when resizing needs done I would think a custom tapered sizing die would be a valuable asset. Even a hand die where the hull is simply driven in & out should be quite practical.
