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 Magnum is nearly 1/10th of an inch longer than the 223 in overall case length and is also slightly longer it body length. As a result, the two are not interchangeable, and although the 223 can be chambered and fired in a 222 Magnum rifle, a dangerous headspace condition exists and case rupture is certain to occur when the round is fired. 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. It never achieved the popularity of the standard 222 and has been largely superseded by the 223 Remington. It is, nevertheless, a very fine long range varmint cartridge. Remington still manufactures ammunition in this caliber.
Source: Cartridges of the World
