Question: 5 mm is what caliber?
Answer: 17 HMR (right). The 5mm Remington Rimfire Magnum or 5mm RFM is a bottlenecked rimfire cartridge introduced by Remington Arms Company in 1969. Remington chambered it in a pair of bolt-action rifles, the Model 591 and Model 592, but the round never became very popular, and the rifles were discontinued in 1974.