Ok, I have a few questions. First, what does ACP stand for? Second, how do you know anything about a bullet by getting its caliber or diameter in mm? For example you have a 7.62mm round, how do you know it will fit your rifle? I know they do stuff like 7.62mm x 54mm or something like that, but what about for pistols? Like for .45 caliber, is there only two types of .45 rounds, magnum and regular? Thanks guys.