Awesome thread Janq. This is all pretty basic knowledge that I'm sure many of us take for granted, but is invaluable for those just starting out.
Both are measurments of the diameter of the bullet. .45 is .45 of an inch, or 45% of an inch in diameter. A 9mm is just that, 9mm in diameter. In gerneral the reason you will see these two different types of measurements is because ammo types that are traditionally american will have the .40, .223, .45 measurement etc., while the eupropean ammo types will measure in mm such as 9mm, 7.62mm, 8mm etc. Think KPH vs. MPH as a speedometer reading. Sometimes the American and Euro measurements will be nearly identical such as 7.62 and .30, or .40 and 10mm, though case lenght will vary.poon said:i have a question since i don't know much about pistols.
in the case of a .45 caliber pistol, what is .45 measuring, or rather what is it measured in?
also, in a 9mm pistol what is it measuring? the diamter of the barrel? thanks .