This is what Bro. Compton said, which I totally do not understand

p. 129 The other variable which can be altered in the test barrel to change the internal ballistics is the chamber length. In Table 3.10 the standard deviation and pellet counts at 40 yards are given for a 36g load of #4 lead shot loaded into a 2 3/4” cartridge cases which were fired through a selection of chambers lengths. The averaged results show a definite alteration to the lateral dispersion of the shot cloud when using the wrong chamber length. The tighter pattern generated by 2 1/2” chamber length may be caused by the restrictive crimp opening acting like an internal choke.

OK Bro. Don; you're on wink

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