Basically you have 4 or 5 factors - diameter, bullet shape, velocity and weight. Velocity and weight get combined into energy. The old Hatcher formula was
RSP = (WB * V * A * F) / 1000
where
RSP = Relative Stopping Power Index
WB = the bullet weight in grains
A = Area of the bullet in in2
V = the bullet velocity in feet per second (f/s)
F = Form factor derived from the chart.
F factor:
Fully Jacketed Pointed .7
Lead Flat Point (Large Flat) 1.1 - 1.2
Fully Jacketed Round Nose .9
Lead Semi-wadcutter 1.25
Lead Round Nose 1
Lead Flat Point 1.05
Jacketed Softpoint (unexpanded) 1 - 1.1
Fully Jacketed Flat Point 1.05
Jacketed Softpoint (expanded) 1.35
Fully Jacketed Flat Point (Large flat) 1.1
Hollow Point (unexpanded) 1.1
Hollow Point (expanded) 1.35
It was a decent attempt, but it got silly if taken to extremes. A baseball thrown at 90 mph comes out with more stopping power than a .45. The coefficients for form factors are completely arbitrary. Does anyone really believe that, all other things being equal, a lead semi-wadcutter has almost the same stopping power as an expanded hollow point?
But the .40/10mm chart I posted was comparing apples to apples - .4" diameter 180 grain bullets. The only difference was velocity. And a lot of the .40 S&W show the same velocity - and therefor the same total energy - as the 10mm.
You can argue over whether a .45 FMJ with less energy has the same stopping power as a .40 FMJ with more energy, I don't have the answer to that. But I think it's obvious that a bigger, heavier bullet will have more stopping power than a lighter, smaller one moving at the same speed. And I think it's obvious that 2 bullets of the same shape, diameter and weight moving at the same speed will do the same damage, no matter what's stamped on the case head.
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