The 222 Remington Magnum was originally developed as an experimental military cartridge in a cooperative effort between Remington and Springfield Arsenal. Since it was never adopted by the military, Remington introduced it as a sporting round in 1958 as one of the calibers for their Model 722 bolt action rifle, and also for a time in the later 700 series bolt action rifles. In comparison to the standard 222 Remington, the magnum version has about 20% greater case capacity, and consequently delivers 100 or so fps higher muzzle velocity and an effective range between 50 and 75 yards greater than the 222. Though its case is 4 to 5 % greater than the 223 Remington, the performance of these two is indistinguishable because the 222 Remington Magnum is factory loaded to a lower maximum pressure. The 222 Remington Magnum is every bit as accurate as the standard 222 or the 223 and is certainly adequate for anything up to but not including deer. 

This text is based on information from “Cartridges of the World”, Hodgdon reloading manual, the cartridge designer and/or own resources.