I always used (weight x velocity x Diameter)/7000(grains per lb) to get my bullet power ratings. its not perfect but it does give me a good comparison, think it was in a "cartridges of the world" book but i cent remember for sure.
I believe that's what's known as the Taylor Knock Out Value or just Taylor KO. Developed by John "Pondoro" Taylor, best known for his big game hunting/poaching in Africa.
Obviously it favors large bore cartridges.
Interestingly (to me anyway), Hornady has a sort of cartridge effectiveness calculator that tends to favor just the opposite. It's called Hornady Index of Terminal Standards (H.I.T.S)
. Sounds fancy, but it's just a sectional density*momentum relationship ...specifically, it's (mass*mass*velocity)/(diameter*diameter*700000).
Where as the Taylor KO value probably is intended to predict wound channel width and rate of energy dump, the H.I.T.S. value seems to be more an indicator of penetration. Of course, neither can quantify bullet materials/design, which can end up making a lot of numbers pointless.