(I'll start here by saying - without question - that bullet construction and placement are critical, regardless of what the external ballistic and impact comparison calculations might prove).
As an engineer that minored in physics in college - and a guy that LOVES shooting and hunting long range - I LOVE formulas and calculations for what I'd call "comparative killing power." And as a guy that LOVES killing power formulas, I've honestly been disappointed with what I found out there. Everything really just comes back to momentum...
Momentum - the basis of Newton's 3rd law - is really the simplest "factor" for any physical system. You know an object's mass, you know it's speed, so without any other complication, multiply those two, and you have something. And because momentum is so utterly basic, it makes sense that most other factors out there are really just a basic momentum number, multiplied or divided by whatever attribute that particular ballistician thought was important.
Guys like Pondoro Taylor and Thornilly noticed that with even with the same momentum, a bigger diameter bullet hit harder than a smaller bullet, so Taylor multiplied momentum by bullet diameter, while Thornilly multiplied momentum by the square root of bullet diameter (and a rather arbitrary factor). Bekker saw value in bullet construction and expansion, so he added multiplied momentum by expanded diameter - and used the RETAINED weight, rather than starting weight of the bullet. Hornady, on the other hand, obviously believes in bullet weight AND penetration, so they multiplied momentum times the bullet weight, then divided by the diameter squared - so a heavier bullet in a smaller caliber will have an advantage in the HITS table. Wooter's Lethality Index, when you break it down, is nothing more than Momentum squared, divided by 450380. Hatcher valued bullet shape and caliber, so he multiplied momentum by a form factor and cross-sectional area. Matuna's thought he was onto something with his Optimal Game Weight formula, by multiplying momentum and Kinetic Energy - but really, kinetic energy is just momentum times velocity and some correction factors, so the OGW formula is really just momentum squared times velocity. The IPSC power factor (also used by several other action shooting sports) is just momentum divided by 1000, and naturally, their power factors are aimed at recoil tolerance and reactive target minimums, so momentum makes a lot of sense...
So as much as I really want to have multiple factors to compare, none of them are REALLY that different - they all rely upon momentum.
