I have a Charles Hellis Premier SLE, 12 bore, 28" bbls with the left having a 7" straight taper to a 0.037" constriction at the muzzle (no parallel). I performs like (surprise, surprise) any other "full" choke constriction. I have found neither any advantage to this long taper, nor disadvantage to lack of parallel.
My model for choke behaviour is that the shot acts mostly like a fluid and is subject to a venturi-like drop in pressure as it passes throuch the choke constriction. If this is so, then a parallel will only subtract from the choke performance due to a friction induce pressure rise within the straight/parallel sided section. I think we can see this as the shot should experience choke effect from passing through the forcing cone, which could be thought of as a choke section followed by a very long parallel. It may be that some forcing cones (ones with about 0.040 constriction) will carry residual choke effect into the muzzle choke and might actually deliver tighter patterns. Generally, we get cylinder patterns from guns with no muzzle constriction, reguardless of the forcing cone passage.
