.17 HMR
Overview
 
.17 Hornady Magnum Rimfire, commonly known as the .17 HMR, is a rimfire rifle cartridge
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...

 developed by the ammunition company Hornady in 2002. It descended from the .22 Magnum
.22 WMR
The .22 Winchester Magnum Rimfire, more commonly called .22 WMR, .22 Magnum, or simply .22 Mag, is a rimfire rifle cartridge...

 by necking down the .22 Magnum case to take a .17 caliber
Caliber
In guns including firearms, caliber or calibre is the approximate internal diameter of the barrel in relation to the diameter of the projectile used in it....

 (4.5 mm) bullet, and it is more costly to shoot than traditional .22 caliber rimfire cartridges. Commonly loaded with a 17 grain
Grain (measure)
A grain is a unit of measurement of mass that is nominally based upon the mass of a single seed of a cereal. From the Bronze Age into the Renaissance the average masses of wheat and barley grains were part of the legal definition of units of mass. However, there is no evidence of any country ever...

 (1.1 g) bullet, it can deliver muzzle velocities in excess of 2550 ft/s (775 m/s).
The .17 HMR round is similar to rounds developed by dedicated rimfire wildcatters
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...

 who worked to create a rimfire cartridge with an exceptionally flat trajectory
Trajectory
A trajectory is the path that a moving object follows through space as a function of time. The object might be a projectile or a satellite, for example. It thus includes the meaning of orbit—the path of a planet, an asteroid or a comet as it travels around a central mass...

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