Calibre: American standard: bullet groove diameter, uses inch convention. European standard: bullet land diameter, uses metric convention. Cartridge: You have many American rounds that have simply the calibre and designer, such as Remington, Winchester, Marlin, Hornady, PPC, Casull, etc. You've got many older rounds which used the convention of xx-yy or xx-yy-zzz, where xx is the calibre, yy is the powder charge, and zzz is the bullet weight in grains. Such as .45-70 gov't or .45-70-450, or .30-30 winchester, or .32-20, .25-20, .38-55, .38-40, .44-40, etc. The dashes are not pronounced; for example ".45-70" is just "forty five, seventy", not "forty five dash seventy". The gov't on the end of that one designates it as an official government (U.S. military) round. You've got European and military rounds with the convention of aa x bb mm, where aa and bb are both millimeter measurements, and aa is the bullet land diameter, and bb is the cartridge case length from back to front, such as 7x57mm mauser, 6.5x55mm swede, 7.92x57mm mauser, 7.62x51mm, 5.56x45mm, 7.62x39mm russian, etc. The "x" in these is read "By", as in "seven by fifty seven millimeter mauser", etc. Then you've got some oddball ones that start with the bullet diameter in inches or mm, but then use a random designation which one must learn on a case-by-case basis, such as .30-'06 springfield, which means .30 caliber (actually .308 caliber), adopted officially in 1906 in that particular loading, by the U.S. government as the standard small arm/rifle round, and springfield added on the end because the U.S. arsenal in Springfield was the place where the round was created/adopted. Quite a few other off-the-wall designations as well. Many rounds have multiple names with both an American and European version of the name. For example, the .380-auto in the U.S. is .380-auto or .380-acp or 9mm Browning short. In Europe it is the 9x17mm or 9mm Kurz. The 38 is actually .357 calibre, like the 357-magnum. The .38 Special and the .357 Magnum do not shoot the same bullet as the 357 SIG however, the 357 SIG shoots .355 calibre (9mm) bullets. The .44 Rem Mag is actually .429 calibre. The .45 ACP is .451 calibre. The .458 Winchester is .458 calibre. Same with .50 calibers. The .50 Action Express pistol cartridge is .50 caliber while the .50 Browning Machine Gun is .510 caliber. You have cartridges named after people, .257 Roberts, .35 Whelen, etc. The .35 Whelen takes .358 caliber bullets rather than the .355 caliber bullets of the 9mm. You have cartridges named after gun magazines, Mr. Simpson's Shooting Times Westerner and Shooting Times Alaskan cartridges; there are cartridges named after the class of weapon they were created for, like the 10mm Auto, and combinations of all of these, like the .32, .380, and .45 Automatic Colt Pistol. The .30-40 Krag was named after the inventor and designates a .30 caliber cartridge with 40 grains of powder. The .30-06 Springfield designates caliber and year adopted instead of powder. Conversion can be an interesting venture also: (These measurements taken from list of ammunition that has both Metric and Standard sizes) Measurements rounded to nearest thousandths. 6mm & .243 calibre calculates into .236 6.5mm = .257 calibre calculates into .256 7mm = .284 calibre calculates into .276 7.62mm = .308 calibre(except Russian, it is actually .311 calibre), calculated it's .300 calibre. 8mm = .323 calibre calculates into .315 8.6mm = .338 calibre calculates into .339 9mm = .355 calibre calculates into .354 10mm = .40 calibre calculates into .394 11.46mm = .451 calibre calculates into .451 12.7mm = .50 calibre calculates into .50 After doing conversions though, you will start noticing some inconsistencies. To convert Standard(inch) to Metric(mm), multiply 25.4(number of mm in inch) by the number in inches(don't forget the decimal; example: 25.4mmx.50cal=12.7mm). To convert Metric(mm) to Standard(inch), divide the Metric size by 25.4(example: 12.7mm divided by 25.4mm= .50in.) In short, the designations given by the manufacturer are just approximations. Hope this helps anyone who needs to know how to convert ammo sizes, and for anyone who didn't know which same sized bullets were actually same size, or different. -Mike.