[BattleRifleG3]

Here's a basic definition of caliber:
When you see numbers like 22 caliber and 30 caliber, that means roughly .22 and .30 inches. When you see numbers like 6.5mm and 8mm, that means something roughly similar in milimeters.
But it gets a little more complicated...
Technically caliber means width of the bullet. But it tends to be used for different cartridges as well. Also, people have a way of estimating a little too much in their terminology, and if you don't know what they're talking about, you may get confused.
Example:
7.62mm = 0.300000 inches. But the term 30 caliber is used to include practically everything under 8mm. Most 30 caliber bullets are .308"wide (.30-30 Krag, .30 carbine, .300 Savage, .30-06, .308, .300 Win Mag, .300 Weatherby Mag, .300 Remington Ultra Mag, 7.82 Lazzeroni Warbird, etc...) Some are not, however, some are .311 (7.62x39mm Soviet, 7.62x54mm Russian, .303 Enfield). Also, beware the metric designations. 9mm Luger and .380 Auto are both .355", while 9mm Makarov is .364". Some numbers are way off, like .444 Marlin and .44 Rem Mag are .429" Some are right on, like .40 S&W (far as I know).
Next, when you see a military cartridge, numbers may look like 7.62x51mm NATO or 8x56R. The first number is the bullet width, the second is the length of the empty case. Some cartridges have other explanations. .30-06 Springfield means 30 caliber, adopted in 1906. .30-30 means 30 caliber, with 30 grains of powder (not sure whether that number's for black powder or smokeless). 250-3000 means 25 caliber, moving at 3000fps. 22-250 means 22 caliber, based on the 250-3000 case.
Basically, cartridge designations are names, not specs (usually).