for small arms, caliber refers to diameter/inside measurement of barrel groove to groove which is usually the outside size of bullet. Caliber is nominal size and certain current arms may have a 'old' caliber/size designation. Example, the .38 caliber pistol now has a .357 bullet in a .376 to .380 diameter cartridge case- go back to early black powder cartridges in transition from cap and ball and you'll find the Colt using a 38 size bullet and nominal .36 size barrel in the Navy pistol. the metallic cartridge had a bullet and case similar to the .22 long rifle, .22 short rimfires of modern times- same size bullet as case. A bit later around time of smokeless powder the bullet was 'inside lubed' and fitted into case , a difference of 2 thickness of case wall then or 1/2 mm or say .015 to .020 in thousands of inch - the .38 case had a .36 nominal bullet for same cylinder chamber and the later pistol had reduced barrel size for smokeless powder. The .44 Henry as outside lube was closer to .45 bullet, barrel rifling was .44 - the 44 S&W american, Russian and special had inside lube and the now noted .43 diameter bullet that was inside lubed. .38 S&W pistol cartridge is metric nominal 9.2 or 9.3mm or .363 in bullet size with a cartridge case of .385. in this case the S&W used the nominal .36 bullet and made the cartridge case bigger to handle the inside lubed bullet. The .38 S&W Special was developed from the Colt cartridge and bullet--SO, the S&W name appears on 2 different cartridges, one developed for the S&W Pistol and second cartridge as 'special' was a improvement of Colt cartridge that had proven a bit weak for military us- S&W increase case length a bit so new, higher pressure cartridge wouldn't fit in 'old' colt revolvers but would fit in newer longer cylinder revolvers and would fire older colt 'long' cartridge and the even older outside lubed from black powder days 'Short'. .38 Colt. The .32acp is a .308/31 diameter but back of case semi rim is .32 as derived from old revolver cartridge that had started as outside lube with same bullet size as case. Metric is 7.65 designation with case length of 17mm. Metric 9mm Luger is .355 nominal bullet- but 9mm Markov/Russian is .363 like S&W or 38 Enfield/Webley British service pistol. Markov ammo boxes usually marked 9.2 or 9.3 x18 for Markov pistol or P1. .38 ACP for the Colt 1911 as Civilian/Police has the .358 nominal bullet size instead of .355 of '9mm' and a straight case of .385 diameter instead of slight taper of 9x19 Luger/Parabellum. .38 ACP was used in some revolvers also- semi rimmed so extractor 'Stars' had something to grip, was a secondary cartridge for Webley officer pistol. .32ACP was used in some small pocket revolvers as well as the self loaders, would usually fit into revolver chambered for .32 S&W long or earlier black powder 'short' chambering. Other meaning of caliber for 'Big' guns- the multiple of bore size for barrel length. Ask a Navy Veteran the if a 5 inch 38 caliber refers to pistol with 5 inch barrel and .38 bullet? NO, the bore size is 5 inches, the barrel is 38 times that bore size- this gun is Destroyer main gun size , not pistol. artillery may have same size shell/projectile used in short barrel howitzer, medium length field gun and longer barrel anti Tank gun. the Caliber indicating barrel length as multiple of bore will give indication of type. .