I once subcribed to idea that velocity of powder gases exiting behind wad was important to pattern density. The basic idea is that at low gas velocities, gas expanding laterally on release from muzzle would disturb wad and overlying shot less than at high gas velocities.
I did then see some agreement with this in my handloads. With high velocity loads with slow burning powders generally giving less dense patterns than those with medium burn rate powders. However, these were not well controlled tests.
My tightest patterning hand load in 12X76 (Browning Citori full choke) was with 76 mm hulls Winchester CF hulls, 1 3/8 oz of shot, IMR SR7625, one-peice wads with long cushining section and Cu-plated #4 magnum shot. Muzzle velocity was about 1200-1250 fps. Patterns were 85-90% with tight centers from full choke barrel and 70-75% and uniform from half choke barrel, all at 40 yds.
Once I went to heavier shot loads, 1,5 to 1,75 oz of #5 and #4 hard shot, I could only get consistantly decently dense patterns (60-70% and even) by going to classical heavy loads of large shot at low velocities (1000 to 1100 fps). I always suspected that this was partly, at least, because this gun was not designed to give dense patterns with such large shot loads.
I did consistantly find that hard (so called "magnum") shot gave denser (and much more even) patterns than soft (so called "chilled") shot, with Cu plating giving maybe 5% denser patterns. Actually, I could improve patterns of Remington factory 76 mm loads with 1 5/8 oz of shot from 45-55% to 70+% just by replacing factory shot with Lawrance magnum, Cu-plated shot -- shot size in both cases was #4. At least in one barrel, that is, the more open choked one, not in the full choked barrel, for which patterns remained in the 50% range.
