Caliber is measured in 100ths of an inch... thru the maximum diameter of the bullet.
Metric bullets are measured in millimeters...thru the maximum diameter...
For example the 9mm bullet is the same as the .38 Special which is actually not a true 38 hundredths...
The .357 magnum is also not a .38 caliber... but it is the same caliber as the 9mm.
Rifle bullets generally have much bigger cases and powder charges than the same caliber bullet/cartridge for a pistol or a revolver... Unless of course the rifle was designed to shoot pistol cartridges...
Now when you are finished with that puzzle... The are some strange designations such as .30-06 and .308 which shoots the same exact bullets from the same cartridge which has been made shorter and less powerful for the .308...
That actually makes the .308 a 30-06 Short...
And then there's the 250-3000 and the 22-250 (
Nothing like the .22 Long Rifle) and the 30-30 and the .27-06...
EDIT This wasn't meant to be correct... it was meant to be sarcastic...