Originally posted by Angus
Well it boils down to the weight as well, now doesn't it?
A heavier bullet will slow down..slower.
And I don't have the weight comparison. 9mm to 0.45 however don't have a big difference in diameter, and btw, why have modern forces taken up the usage of what, 5.6 mm high velocity rounds?
You see, penetration is also an issue.... a stabbing issue 
Bullet weight, measured in grains, comes into play here.
Most early and standard 9mm. ball, or FMJ, rounds are around 115-125 grains. The .45 ACP FMJ weighs 230 grains, about twice as much. That extra weight translates' into more 'carry through' energy on-target.
BTW, a 9mm projectile measured in standard english measurement is about .357 of an inch. (So is a .38 special projectile; The reason for that goes' back to black-powder days). It may not seem like much, but when you increase the diameter of a .35 round circle by an extra .10 inch, it makes quite a difference.
As to the 5.56mm., In SAE measurement, it is known as a .223, or .22 if you don't want to be so picky with the micrometer. The theory behind the 5.56mm, ever since It wa introduced in the late '50's, is that a 55-grain bullet travelling at 3,500-4,000 FPS would cause wounding, without necessarily killing, Enemy soldiers. DoD studies' had come to the conclusion that a wounded Man was better than a dead one, because wounds' required battlefield medics, Doctors, and nurses to care for him. It was theorized that the logistics' of manpower dedicated to medical support for enemy forces' would be an enormous advantage. Plus, It was also estimated that the average combat-load per soldier could be tripled with the smaller, lighter ammo; Soldiers' that had previously carried about 100 rounds of 7.62mm NATO could now carry something like 300 rounds' of 5.56mm, which also helped improve the logistical situation in the field. It turned out that there were quite a few reasons' to switch.
