I read the linked article, and I won't dispute their results, and I do compliment them on their efforts to use guns of identical weight as much as possible.
HOWEVER,
I think writing that the 9mm has 42% less recoil than the .45 is ...misleading. and that is because of the way we jumble up terms and definitions.
As I read their test, I didn't find that the 9mm had 42% less recoil, what I saw was that recoil moved the 9mm gun in the rest 42% less than the 45 did.
"But, isn't that the same thing???"
yes, and no.
Movement of the gun in the rest isn't recoil, its
work done by recoil. The difference is subtle, but important to understand.
lets define recoil, so we're all on the same page, as :force opposite the direction of bullet travel. (if you prefer a different definition, I'll listen to your reasoning..
)
Now as long as Newton's 3rd law is in effect, recoil energy must be equal to the bullet's energy. (in rough terms, it is a little more complex, I freely admit)
That's the energy
available to do work. How that energy performs work involves many different factors. Time matters. Mass matters. Surface area matters, as well as other things.
The standard ball ammo for the 9mm and .45acp have approximately the same amount of energy. (365ish ft/lbs) That's all the energy available, and its the same, so recoil energy must likewise, be the same.
Movement of the gun is work done by that energy. Felt recoil is our subjective impression of that work, and involves still other factors.
"So, the 9mm did 42% less work?"
again, yes, and no. yes, because the distance the gun moved in the rest was 42% less for the 9mm, and no, because in that comparison, the distance the gun moved was the
only thing being measured.
The distance moved doesn't account for the way the energy was applied, only the end result of that application. Time matters.
Say you have a big heavy door,
slap the door with X ft/lbs of force, the door moves a little bit.
Push the door with the same X ft/lbs and it moves several inches. Same amount of energy, different result. Time matters.
Of course, all this is only somewhat relevant to how the gun feels in your hands when you shoot, and that involves a lot more.