The 223 Remington first appeared in 1957 as an experimental military cartridge for the Armalite AR-15 assault rifle. In 1964, it was officially adopted by the U.S. Army as the 5.6mm Ball cartridge M193. The 223 Remington is nearly identical to the 222 Remington Magnum, the only difference is the 223 has a slightly shorter case. The two are not interchangeable although the 223 will chamber in the 222 Remington Magnum rifle. The result, though, is to create a gross headspace problem, and the 223 case will rupture in the 222 Remington Magnum chamber. As a sporting round, it is just as accurate as any other long range, centerfire, 22's. Military brass cases are usually heavier than commercial cases so maximum loads should be reduced by at least 10% and approached cautiously. That is because the reduced case capacity results in a higher loading density and increased pressure with the same powder charge. The 223 Remington can be classed as an excellent medium range varmint cartridge at ranges out to 250 yards. 

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