The .17 Hornady Magnum Rimfire is a small calibre rifle cartridge designed by Hornady employee Dave Emary (originally as a private pet project) and has been in production since 2002. This rimfire cartridge, more commonly known as the .17 HMR, is based on the larger .22 WMR cartridge, the casing of which is necked down to accept a .17 inch diameter bullet.
The .17 HMR, as mentioned above, was based on the .22 WMR cartridge. The intention had been to create a rimfire cartridge that produced similar ballistic capabilities to the 1970's 5mm Remington Magnum Rimfire (5mm RMR) which had been the highest velocity cartridge in the world at that time. The end of the 5mm diameter (0.20in) bullet meant that the .17 inch diameter bullet came to prominence and the scarcity of the 5mm RMR pushed wildcatters (whom at that stage were leading the push to manufacture the equivalent of the .17 HMR) to use the .22 WMR cartridge as a parent case.
Pressure placed by Sturm, Ruger & Co. and Marlin Firearms pushed Hornady (along with the two rifle manufacturers) into the development process. Like the wildcatters, Hornady decided to use the .22 WMR cartridge as a starting point. After necking the cartridge down, the three companies realised that only a minor modification would have to made to the barrels of .22 WMR chambered firearms to fire the .17 HMR.
The .17 HMR was constructed with a single goal: Replicate the performance of the 5mm RMR cartridge. Not only did the .17 HMR do this, but also massively exceeded the ballistic performance of it's parent case.
|15.5gr (1.0g)||2,525ft/s (769m/s)||236ft.lbf (319J)|
|17gr (1.1g)||2,550ft/s (780m/s)||245ft.lbf (332J)|
|20gr (1.3g)||2350ft/s (720m/s)||
Although these figures are impressive (almost double the muzzle velocity of the .22 Long Rifle, the world's most popular round) it must be noted that the staggering performance comes at a significantly greater cost. That being said the .17 HMR is still cheaper than most centerfire cartridges and has a significantly better ballistic performance than most with a virtually flat trajectory (compared to the .22 Long Rifle or .22 WMR).
http://en.wikipedia.org/wiki/File:17_HMR_-_SB_-_3.jpg - Image Origin