Shotshell ballistics are considerably different than rifle. The variables that effect “power” are only the amount of powder (dram equivalent is an old black powder measure) and the amount of shot. The nominal dram equivalent for most gauges equals a muzzle velocity of about 1200 fps.
The heavier the shot amount and the faster you drive it, the more “powerful” the load is. Of course, it will have heavier recoil.
Also, shot (particularly in smaller sizes) bleeds off velocity very fast. A friend of mine used to shoot 1 ounce at 1350 fps:muzzle. I shot 1 ounce at 1180 fps: muzzle. At 40 yards the difference was only about 50 fps. Plus, a higher initial speed can adversely effect patterns due to air resistance having more effect on deformed pellets.
High brass versus low brass in no certain indication of “power”, look at the powder charge and shot charge weight.
A technical question... I was curious how you measure the velocity of the shot at 40 yards? I have a chronograph that I use to measure muzzle velocity... do you just set your chronograph behind some type of barrier and are then able to catch the shadows of pellets flying by 40 yards away? I have just never tried it and was curious how you set up to do it.