[collegeburner]

Because there's a lot more than caliber that matters. If you think about it, a .45 ACP is close in caliber to a .50 BMG. But there's hella powder behind a .50 and not so much behind a .45. The bullets themselves make a big difference also, there are at least a half dozen you can read more about here: https://gunvault.com/types-of-ammunition/

If you think having 2 cartridges that both have .38" caliber bullets is confusing, take a look at all the 9mm ones. Just in the ones that are actually called 9mm and not just caliber equivalent, there's 9x19, 9x25 mauser, 9x57 mauser, 9x39, 9mm winmag, 9x18 makarov, and a ton more I can't remember. Some of those are rifle cartridges, some handgun. They have different bullets with different characteristics and different bullet lengths and different bullet weights. And the shape of the cartridges behind them, the guns they fire from, and the amount of propellant each one has is all different.