Thank you. But doesn’t high brass have considerable more powder than low brass?
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.
No, the length of the brass doesn't have a lot of predictive value as to the amount of powder being used. I personally prefer the high brass because I reload cartridges and I think that they tend to hold up a little better.