The lack of a parallel section in the choke is not your problem.
Both Fox & Parker used a completely tapered choke for the entire course of their production. If the lack of a parallel section caused blown patterns none of these guns would shoot worth a darn.
Your issue just proves the point that every gun or in this case every barrel shoots differently with any given load. If your bore gauge is one of the dial types try rotateing it at several points in your suspect choke. What you may have is a concentricity problem where the end of the barrel has been dinged or dented in the past and now is oblong rather than a nice round circle.
The other thing to consider here is that one of your barrels has a full taper choke and the other has a parallel section to it. This tells me one or the other chokes has been altered at some time. I suspect that the parallel one is the one that has been altered. If this were my gun I would get it to a choke specialist like Mike Orlen, tell them what your experiencing and follow thier recommendations to correct the problem.
