Why was the 45 caliper gun invented?

Answer 1

I presume the question is asking about the US .45 pistol round (the British Martini-Henry rifle also fired a bullet of .45 caliber). In the 1800's, the Colt pistol was made in .45 probably because it was a big round which typically struck the target with enough force to require only one shot. When the US assumed control of the Philippines from Spain in 1898, the US Army was using a pistol in .38 Long, and there were complaints that several shots were required to stop Filippino rebels, so the US Army returned to the .45 for about one more century before exchanging it for the 9mm.