It's the same. The caliber of a bullet is its diameter.
A .222 caliber bullet has a diameter of 5.56mm.
The caliber of a bullet, refers to it's diameter in inches. But sometimes other measurements are used, such as mm.
A 22 caliber bullet is 22/100 inches in diameter. A 7.62 mm bullet is 30 caliber or 30/100 inches in diameter there is no such thing as a 7.62 caliber bullet
metric vs inch measurement
Short answer, a 9mm is smaller in diameter and lighter.
As far as diameter, there is very little difference. A .357 is .358 in diameter and a 9mm is .356 in diameter.
In the usual sense of caliber, it means the diameter of the bullet. The smallest commercial caliber is the .17 rimfire, but there have been MUCH smaller caliber firearms, down to the 1-2 mm range. That is about .05 to .10 caliber.
The caliber of the firearms. This is the bore diameter given in fractions of an inch, or in millimeters. A .30 caliber rifle is about .30 inches in diameter. An 8mm rifle is about 8 mm in diameter. Shotguns use an entirely different system known as gauge.
They are different ways of expressing the same thing. A bullet that is 7.65mm is just that- 7.65 millimeters in diameter. Is the US, we frequently use caliber rather than millimeters- that same bullet was be a .32 caliber- or 32/100ths of an inch in diameter.
The circumference of a circle with a diameter of 55 mm is: 172.8 mm
Caliber is in reference to the diameter of the bullet. A .40 (which is what I have) is .40 inches in diameter. A .50 caliber is .50 inches in diameter (half an inch). So a caliber is the inches in diameter. Not including the 9mm which I cannot stand