What do the letters mean that follow the numbers on ammunition?

While most people realize the numbers on their ammunition represent the caliber or diameter of the cartridge, the letters preceding them can be a bit mysterious.

Most often these acronyms are placed there by the manufacturer that invented the cartridge, and are used to identify the type and inventor of the round. Here is a list of some commonly used acronyms on ammunition and their meanings-

ACP = Automatic Colt Pistol (Examples: .32 ACP, .380 ACP, .45 ACP)

WCF = Winchester Centerfire (Examples: .25 WCF (.25-20), .38 WCF (.38-40), .44 WCF (.44.40), .30 WCF (.30-30) 

WMR = Winchester Magnum Rimfire (Example: .22 WMR or .22 Magnum)

HMR - Hornady Magnum Rimfire (Example: .17 HMR or .17 Hornady Magnum) 

WSM = Winchester Short Magnum (Examples: .270 WSM, .300 WSM, 7mm WSM)

WSSM = Winchester Super Short Magnum (Examples: .223 WSSM, .243 WSSM, .25 WSSM) 

RUM = Remington Ultra Magnum (Examples: .300 RUM, .338 RUM, 7mm RUM, 375 RUM)