Mustang is correct, as I recall
as I understand it, switched because of less weight, can carry more ammo in the field. As to why the switched to 9mm on the handgun caliber has more to do with NATO pressure then best bullet/best caliber, as I understand the history. Naturally every police force jumped on the 9mm craze back then , then the civilians did since it must be the best caliber, heck the army and the police force switched...I wonder how many police force's still use the 9mm, think most have dumped it in favor of the 40. Moral of the story, just because the armed forces picks something doesn't make it the best, or at least, the best for you. In my house, doubt if any bad guy is going to let me shoot at him 15 times so I'd opt for a .45, out on the street where you may encounter several gang bangers, well the additional rounds may be better then the lower rounds of a .45.
My two cents worth, and my history of the military's changing of both of the above mentioned rounds, as I recall the history.