.17 Rimfire may refer to a number of different cartridges

Cartridge (firearms)

A cartridge, also called a round, packages the bullet, gunpowder and primer into a single metallic case precisely made to fit the firing chamber of a firearm. The primer is a small charge of impact-sensitive chemical that may be located at the center of the case head or at its rim . Electrically...

 or firearm

Firearm

A firearm is a weapon that launches one, or many, projectile at high velocity through confined burning of a propellant. This subsonic burning process is technically known as deflagration, as opposed to supersonic combustion known as a detonation. In older firearms, the propellant was typically...

s that use them. The cartridges all use rimfire cases and fire .17 inch (4.5 mm) bullets. While .17 caliber airguns generally use .177 inch projectiles (4.50 mm), .17 caliber firearms, both rimfire and centerfire, generally measure .172 inch (4.37 mm).

.17 Rimfire cartridges include:

• .17 HMR
  .17 HMR
  .17 Hornady Magnum Rimfire, commonly known as the .17 HMR, is a rimfire rifle cartridge developed by the ammunition company Hornady in 2002. It descended from the .22 Magnum by necking down the .22 Magnum case to take a .17 caliber bullet, and it is more costly to shoot than traditional .22...
  
  , or .17 HMR, the first major production .17 caliber rimfire, introduced in 2002
• .17 HM2
  .17 HM2
  The .17 Hornady Mach 2, or .17 HM2, is a rimfire cartridge introduced in 2004 by the ammunition manufacturer Hornady, following the successful launch in 2002 of the .17 HMR...
  
  , or .17 MH2, introduced in 2004.
• The .17 PMC/Aguila
  .17 PMC/Aguila
  The .17 PMC/Aguila or .17 High Standard is a rimfire cartridge formed by necking down the .22 Long Rifle casing to accept a .172" diameter bullet. This cartridge was developed in 2003 by firearms maker High Standard and ammunition maker Aguila and introduced in 2004...
  
   introduced as .17 High Standard and .17 Aguila in 2003/2004.
• Any of a number of .17 caliber wildcat cartridge
  Wildcat cartridge
  A wildcat cartridge, or wildcat, is a custom cartridge for which ammunition and firearms are not mass produced. These cartridges are often created in order to optimize a certain performance characteristic of an existing commercial cartridge.Developing and using wildcat cartridges does not...
  
  s based on rimfire cases. Most of these were, like the .17 HMR, based on the .22 WMR
  .22 WMR
  The .22 Winchester Magnum Rimfire, more commonly called .22 WMR, .22 Magnum, or simply .22 Mag, is a rimfire rifle cartridge...
  
  .