.17 HMR

.17 HMR
17-grain (1.1 g) A Sellier and Bellot .17 HMR round with a light green plastic tip.
Type	Rifle
Place of origin	United States
Production history
Designer	Hornady
Designed	2002
Manufacturer	CCI, Federal, Hornady, PMC,Remington, Winchester
Produced	2002–present
Specifications
Parent case	.22 WMR
Bullet diameter	.172 in (4.4 mm)
Neck diameter	.190 in (4.8 mm)
Shoulder diameter	.238 in (6.0 mm)
Base diameter	.238 in (6.0 mm)
Rim diameter	.286 in (7.3 mm)
Case length	1.058 in (26.9 mm)
Overall length	1.349 in (34.3 mm)
Primer type	Rimfire
Ballistic performance
Bullet mass/type Velocity Energy 17 gr. (1.1 g) 2,550 ft/s (780 m/s) 245 ft·lbf (332 J) 20 gr. (1.3 g) 2,350 ft/s (720 m/s) 250 ft·lbf (340 J)
Source(s): Hornady

.17 Hornady Magnum Rimfire (4.5×27mmR), 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 (4.5 mm) projectile. Commonly loaded with a 17 grain (1.1 g) projectile, it can deliver muzzle velocities in excess of 775 m/s (2550 ft/s).

The .17 HMR round is similar to rounds developed by dedicated rimfire wildcatters who worked to create a rimfire cartridge with an exceptionally flat trajectory. These wildcatters were seeking to match the ballistics of the obsolete 5mm Remington Magnum Rimfire, which was made from 1970 to 1974, and was to that point the fastest rimfire cartridge ever produced. With 5mm (.20 caliber) diameter barrels and bullets being virtually unavailable at the time (the 5mm RMR was the last commercial 5mm round until the 2004 release of the centerfire .204 Ruger), the commercially available .17 caliber became their bullet of choice. The .22 Winchester Magnum Rimfire was the logical parent case, rather than 5mm RMR (with its unique case head size, which requires a significantly different bolt and magazine), because it was commonly available, and it is a far larger and stronger case than the next largest .22 Long Rifle. The .17 caliber wildcats not only met, but far exceeded the 5 mm RMR's velocities and flat trajectory. The accuracy of these cartridges was also quite good. However, the downrange energy of the 5mm RMR is superior to both .22 WMR and .17 HMR, so there is still potential in the 5mm rimfire for wildcatters.

Hornady, in conjunction with Marlin Firearms and Sturm, Ruger (manufacturers in the rimfire rifle market), followed much the same path. With the .22 WMR case as the starting point, a simple barrel change was sufficient for most .22 WMR firearms to chamber the new cartridge. In 2002 the first rifles and ammunition began appearing on the market. While the ammunition was relatively expensive due to the high-performance .17 caliber bullets used, it was still cheaper than most centerfire ammunition. By 2004 CCI, Federal Cartridge and Remington had each introduced .17 HMR ammunition offerings.