Okay, so many years ago (hence me being 15 in the picture), I went to the range with my grandfather. Of several firearms I used (9mm S&W, 9mm luger, 38 special S&W, and 12 gauges w/slugs), I fell in love with his 6mm, single-shot, bolt-action rifle. Now, I know that 6mm is a strange cartridge, and when I search for it online, I cant find a damn thing. My question is, what exactly is this cartridge, when was it created, and for what? I can tell you that the gun had considerable kick, more than a 7.62x39mm, the cartridge is big, about .25 inches smaller than a 12 gauge in height, my grandfather inherited the gun from a dead friend, and that the ammo was remington. Thanks!

^note: disregard my hair. I was young and stupid.