I worked with a guy who had been part of the selection committee at the time the 9mm was selected. He had been in the Air Force, and this was the cold war era....so they fixated on the pistol’s ability to kill a polar bear...in the event a U2 pilot or something similar had to bail over the arctic after spying on the Ruskies.
So, it became a game of “how small can we go, and still kill a polar bear”. Apparently the break point is 9mm. I have no idea how this was tested, or where on earth you would shoot a polar bear, but allegedly the 9mm will do the trick. And, since a polar bear is bigger than a man, it was assumed the 9mm would be perfectly good for killing men too...nobody accounted for body armor, etc.
Why were they looking to switch from the .45 in the first place? First, I remember being trained on the 1911...even while taking it apart and doing function checks, etc. it became obvious that our 1911 inventory was old, loose, and worn out. There wasn’t an option to just keep what we had - a purchase had to be made. As an example, I first hit the range with the M9 and scored expert...as did 90% of the people in my group. I never scored expert with a rifle, but did it right out of the gate with the pistol. It was obvious that the pistol range (pop up targets) had been set up for the Army’s inventory of worn out and inaccurate 1911’s...and it was almost too easy with an M9. Just another indicator that any pistol would be an improvement over the current inventory.
So why the 9mm? For starters, NATO was causing a standardization of calibers. Also, the 9mm holds more rounds. And I think this one is important - higher rate of fire. I have tested this many times on paper targets - I can get accurate shots off more quickly with the 9mm than I can with a .45. Its just a matter of recoil.
I liked the M9 when I was in the Army enough to own a civilian version today. So I was satisfied...and honestly never too concerned about its stopping power. I was a tank commander, so this was my personal ‘oh crap’ weapon...its not like I was planning on kicking down doors and clearing rooms with it - which may have given me a different perspective.
I think the Army may be leaning towards a .40 caliber. Some federal agencies are already using this, and it seems to hit the ‘sweet spot’ between mass and velocity, as far as stopping power goes. I don’t see many police departments, or federal agents using a .45....so I really have no expectation that the Army would go back to that, or anything like the 1911.
The G22 is already being built in America, so that pistol would be a nice, quick fix. The recoil on the full-sized G22 is amazingly manageable.
I think it depends on the 45. With my Kimber, I can put rounds in the center of a target at 25 meters as fast as I can pull the trigger, recoil is slight, the pistol has weight.