| 9x19mm(left) and .45 ACP(right)
The .45 ACP is a pistol cartridge developed in 1904 by John Browning for his prototype semi-automatic pistol. This pistol was later adopted by the U.S. Army as the Colt M1911, gaining both the gun and the cartridge instant widespread usage.
Firing a large 230 gr. projectile at a relatively slow 850 fps(feet per second), the .45 ACP has developed such a reputation for stopping power that after 95 years it is still the standard issue pistol caliber for U.S. special forces(H&K Mk.23) and many law enforcement agencies.
Aside from being a pistol cartridge, the .45 ACP has also been used in may submachineguns for it's close range accuracy and performance against living targets. Invented shortly after WWI the Thompson Submachinegun(aka. "Tommygun", "Chicago Typewriter") was fielded by both law enforcement and gangsters alike. With the onset of WWII, army non-commissioned officers and patrol leaders were issued the Thompson SMG.
Heckler & Koch used the .45 ACP for their UMP45 submachinegun as a replacement for the 9mm MP5(though both are still in production).