Understanding caliber
Confused? You will be. 
Trying to explaincaliber sizing to someone who knows little about firearms can be a bit of a challenge mainly because it really isnt a standard convention anymore. I thought I would put together a short article on how, why and the differences in naming calibers.
In short, caliber isthe measurement of the internal bore of a barrel. And that is about where the simple description ends. You see, because most modern barrels have rifling, there is actually two places you can take this measurement from. The lands, which are the high points in the rifling, or the groove,which are the low points. Generally, the US is going to measure from the groove, while the rest of the world will measure from the lands.
Generally, where acaliber bore measurement is given in inches, the inches can simply be replaced with cal. For example .308 cal is a bullet generally .308 inches in diameter.
However, imperial isntstandard in a lot of the world, so metric can also be used. For example, the .308s close relative, the 7.62×51mm NATO is a 7.82mm projectile in a 51mm lengthed case for a barrel with a land to land bore of 7.62mm. However, because just using one or the other would be too simple youalso have manufacturers putting out hybrids i.e. the 7mm-08. This is a .308 (imperial) cartridge, necked down to fit a 7mm (metric) bullet.
Then we have wildcatand homebrew cartridges that really just get named however the maker wants.
Basically, we are at apoint where we need a chart to make sense of it. This one comes straight from Wikipedia.
https://en.wikipedia.org/wiki/Caliber
Common calibers in inch and their metric equivalents
| Inch caliber | Metric caliber | Typical bullet diameter | Common cartridges | Notes |
|---|---|---|---|---|
| .20, .204 | 5 mm | 0.204 in | .204 Ruger, 5 mm Remington Rimfire Magnum | |
| .22 | 5.56 mm | 0.2200.224 in (5.65.7 mm) | .22 Long Rifle, .222 Remington, .223 Remington, 5.56×45 mm NATO, 5.45×39 mm, 5.7×28 mm | 5.45×39 mm bullet is actually 5.6 mm (AK-74) |
| .24 | 6 mm | 0.243 in | .243 Winchester , 6 mm Remington, 6 mm plastic airsoft BBs | |
| .25 | 6.35 mm | 0.25 in, 6.35 mm | .25 ACP , 6.35×16 mmSR | or .25 auto and 6.35 mm Browning |
| .26 | 6.5 mm | 0.264 in, 6.7 mm | 6.5×55 mm, .260 Remington | cartridges commonly known as 6.5 mm |
| .27 | 6.8 mm | 0.277 in, 7.035 mm | .270 Winchester, 6.8 SPC | |
| .28 | 7 mm | 0.284 in, 7.213 mm | .280 Remington, 7 mm Remington Magnum, 7×57 mm, 7mm-08 Remington | commonly called 7 mm |
| .30, .308 | 7.8 mm | 0.308 in | 300AAC Blackout, .30-06, .300 Winchester Magnum, 7.82 Lazzeroni Patriot,.30-30 Winchester, .308 Winchester, 7.62×51mm NATO | American .30 caliber |
| .303, .31 | 7.9 mm | 0.310.312 in (7.97.9 mm) | .303 British, 7.62×39, 7.62×54R, 7.62×25, 7.7×58 | 7.62×39 rounds are actually 7.92 mm (AK-47, AKM, etc.) |
| .323 | 8 mm | 0.323 in | 8×57mm IS , .325 WSM, 8 mm Remington Magnum, 8 mm plastic (airsoft) BBs | .32 caliber rifle cartridges |
| .338 | 8.6 mm | 0.338 in | .338 Lapua | C14 Timberwolf (Canadian Forces) |
| .357 | 9 mm | 0.3550.357 in (9.09.1 mm) | .38 Special , .380 ACP, .357 Magnum, .357 SIG, .35 Remington, 9×19 mm Parabellum, 9×18 mm Makarov, .357 in certain new Crosman precharged pneumatic (PCP) airguns. | Handgun cartridges known as 38 are .357 caliber. Generally .357 for revolvers and rifles, .355 in autoloaders |
| .40 | 10 mm | 0.400 in | .40 S&W, 10 mm auto | |
| .45 | 11.43 mm | 0.450 in | .45 ACP, .45 GAP, .454 Casull, .45 Long Colt | |
| .50 | 12.7 mm | 0.510 in (12.95 mm) | .50 BMG, .50 Action Express, 12.7×108 mm | M2 Browning machine gun and other heavy machine guns, long-range rifles typified by Barrettproducts |