It’s hard to come up with a more iconic American handgun cartridge than the .45 ACP (Automatic Colt Pistol). Best known for the U.S. M1911A1 pistol made famous during WWII, there are still countless handguns in modern production that use 45 ACP ammo, including many variations on the original 1911 blueprint. There are also carbine rifles that are chambered for the 45 caliber ACP round, like the overwhelmingly popular AR platform. Putting 45 auto ammo through a rifle has the advantage of longer accuracy and reduces felt recoil down to next-to-nothing compared to a pistol.
Although not the only 45 round, 45 caliber bullet, or 45 bullets available, .45 ACP ammo is definitely the most popular caliber in this size