ORIGINAL: stubblejumper
A caliber is simply the bullet diameter in inches.The 308win,30-06 and 300win mag are all the same caliber which is .308".The 270win is actually .277" cal.On the other hand cartridge takes into account the case as well as the bullet diameter.For example the 30-06 is a cartridge not a caliber.
Exactly. In other words, .308 caliber is about 1/3 of an inch. 9mm. means exactly that 9millimeters.
.308 caliber can also be expressed as 7.62mm. That only refers to the width of the bullet. You also take into account the length of the bullet and the amount of powder behind it.