I was going to hijack Surpip's mossberg thread, but thought that was rude... At the risk of sounding like an idiot... I'm having issue's understanding caliber. I understand metric (9mm, etc), but I'm not really grasping the measurement technique that makes a S&W .500 not the same as a .50, but the .223 is similar in diameter to a .22 (yes, I know they're completely different rounds). I know they're all very different size/power rounds, that's pretty obvious from the rounds themselves, etc, but how is caliber measured that zero's after the decimal matter? My original understanding of caliber was that it was a very old measurement system that measured the diameter and number of ball bearings that could fit across a breech or something like that (I barely remember it).