John Browning, working for the weapons manufacturer Colt, was the mastermind, but the .45 caliber wasn’t his first option. In fact, he had been working on a .41 caliber before the military decided, after testing several caliber sizes, that the .45 was the proper size. It started out being developed with a 200 grain bullet that was able to reach a velocity of 900 feet per second.
However, after more intense testing, the round was adapted to use a 230-grain bullet and go on to become known as the .45 ACP. Though the round is still extremely popular, and used by many agencies around the world, many are still switching to the 9mm strictly based on cost and the amount of rounds available in the gun.