Claude Purbaugh, the originator of sub-gauge tube inserts, told me the parallel section should be long enough to contain the entire shot charge. He did not distinguish between the degree of choke, nor do I know if he had ever proven this theory in any manner. Most of what I've read suggests that some parallel gives the best(whatever that means) patterns, but also if the parallel section is too long(whatever that means) the choke will produce less, maybe no constriction. This also suggests, to me, that inward deflection or acceleration produced by the approach taper is what makes choke work in the first place. JL
