Remington designed a completely new, bottlenecked case that was somewhat similar to the older .22 Winchester Magnum, but stronger to handle the higher pressure of the 5 mm at 33,000 PSI.
It uses a 5 mm (.204 caliber) bullet that measures 0.2045 inches (5.19 mm), the same as the more recent .204 Ruger center fire cartridge.

From 1982 until 2008, no ammunition manufacturers manufactured rounds for this cartridge.
Some firearms manufacturers even created conversion kits to allow the existing 5 mm guns to shoot other more-common cartridges.

At the 2008 SHOT Show, Aguila Ammunition announced it would reintroduce the cartridge and begin commercial production of 5mm RFM ammunition.
Until 2019, the cartridges were sold in the United States of America under the Centurion brand.
In 2019, Aguila Ammunition announced the company now offered two 5mm RFM loads: one with a semi-jacketed hollow point bullet and a second with a full jacketed hollow point bullet.
Both loads use a 30 grain projectile and have a muzzle velocity of 2,300 fps.
Remington designed a completely new, bottlenecked case that was somewhat similar to the older .22 Winchester Magnum, but stronger to handle the higher pressure of the 5 mm at 33,000 PSI. It uses a 5 mm (.204 caliber) bullet that measures 0.2045 inches (5.19 mm), the same as the more recent .204 Ruger center fire cartridge.
From 1982 until 2008, no ammunition manufacturers manufactured rounds for this cartridge. Some firearms manufacturers even created conversion kits to allow the existing 5 mm guns to shoot other more-common cartridges.
At the 2008 SHOT Show, Aguila Ammunition announced it would reintroduce the cartridge and begin commercial production of 5mm RFM ammunition. Until 2019, the cartridges were sold in the United States of America under the Centurion brand. In 2019, Aguila Ammunition announced the company now offered two 5mm RFM loads: one with a semi-jacketed hollow point bullet and a second with a full jacketed hollow point bullet. Both loads use a 30 grain projectile and have a muzzle velocity of 2,300 fps.
5mm Remington Rimfire Magnum