There is a body of thought backed up with a fair amount of tests by trap shooters and articles that says that Green Dot [comparitively slow] gives slightly higher percentage pattern yeilds than does Red Dot [comapratively faster] when using the same shot in similar velocity loads. I never did that particualr comparison as I never much used Red Dot powder. I have found that slightly lower velocity loads will generally yeild tighter patterns and that the claimed amount of Antimony matters little once the shot size reaches #5 in uncoated lead shot. I generally use published data w/some primer switching at times [at my own risk] & I had/have no means of measuring pressures aside from sending some few rounds off to be tested, but that was to insure their safety & not for pattern comparison purposes. Shot hardness does matter in smaller sizes, and that is essentially irrefutable. The rest is a matter of what the plate or paper shows you and your confidence in its truth. Reality is subject to exceptions .. the 'golden pellet'!
Logic & some patterning as confirmation both would indicate that a less abrupt ignition should slightly improve the patterns; logic = less pellet deformation; patterning = slight % improvements. However, it is hard to prove as any absolute. I think in large part because of today's plastic wads that compress a great deal [expand the combustion chamber size, reduce pressure and blow to shot column] before the shot column even begins to move, and even then the shot is protected by the plastic cup from being forced into direct contact w/the bbl. With older card/felt wads I would think the comparison may be more readily seen or distinct.
Going back to published data, I generally use the lower published pressure loads for any given velocity within the 1050~1220 fps range, but have to admit that really only applies to the 12ga., finding reliable 'low pressure' loads for the smaller gauges is often quite hard to do.
Hope you find something here from an old & ever curious shooter of use;-)
