Does anybody have like 100 horses standing around that they don't want anymore?
Because the only way to "scientifically" settle this is to tie them up, shoot them with various bullets, and time how long it takes them to actually die.
You'll want to be close to make sure that shot placement is perfect every time, 50 yards should do it. Use downloaded ammo so the bullets are impacting at long range velocities. Come up with a Garmin to chronograph each shot as it impacts.
Bullets: TMK, FMJ, Mono, and something with a picture of an elk on the box
Calibers: .223, 6.5, and 30 should be enough spread. You don't need multiple rifles, just interpret the data as "This is like a 308 at X hundred yards which is the same as a 300 Mag at 200 yards past that"
That's 12 combinations of bullet and rifle.
Eight horses shot with each combination equals 96. Eight should be enough to be statistically significant, and leaves 4 horses to haul wagons to the glue factory.
How well do these things stick to fur?