system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some ammo types use caliber (.50, .45, etc.) and some use mm measurements (10mm, 5.56mm, 12.7mm, etc)?
gpt
Those are both caliber specifications. .50 cal means a caliber of .5 inches. Things developed in the US used to use inches for caliber, although there's been a shift in some cases to millimeters (among other things, the US military uses metric these days, as does NATO).