We all know that the primary objective of a bullet is to kill or incapacitate its target. On this, the diameter of a bullet plays a significant role.
For me, a big bullet travelling at a moderate speed seems to have a better "stopping power" than to smaller bullet travelling at high speed - even with the same notional muzzle energy.
Compare between .45acp pistol and 9mm Luger cartridges. The 9mm ammo fires a .353in (9mm) diameter, 115 grain (7.45 g) jacketed bullet at approximately 1150 fps (350 mps) giving muzzle energy of 338 ft lb (458 joules).
Some .45 ACP service ammo (hardball, fires a .45in (11.5mm) diameter, 230 grain (14.9 g) bullet of 800 fps (243 mps) for327 ft lb (443 joules).
On the basis of muzzle energy it is claimed that the 9mm Luger is more powerful, yet in accurately documented firefights, the .45 ACP hardball has proved time and time again to be more effective at killing and incapacitating an opponents with fewer shots. Because "BIG HOLES LET IN A LOT OF AIR AND LET OUT A LOT OF BLOOD".
Any comments pls to start a new studies on this two most debated cartridges.