From Inception to Present
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. It is used in the selective fire M16 rifle which is based on the original AR15 design. The cartridge was the work of Robert Hutton, who was technical editor for Guns & Ammo magazine and had a rifle range in Topanga Canyon California. One of the requirements for the cartridge was for the projectile have a retained velocity in excess the speed of sound (about 1080 fps at sea level) at 500 yards, something you could not achieve with the 222 Remington. Working with Gene Stoner of Armalite, Bob Hutton designed a case slightly longer than the 222 and had Sierra make a 55 grain boattail bullet. This combination met the design requirements. All this was documented in the 1971 issue of Guns & Ammo Annual.
Originally an alternate military cartridge, the 223 (5.6 x 45mm) is now the official U.S. and NATO military round. It should also be noted that NATO forces, including the U.S., have standardized a new 5.56 x 45mm round with a heavier 62gr bullet - the SS109, and the M193 is no longer standard.
More recently, in light of experiences with lack of stopping power at extended ranges in Afganistan, SOCOM have been experimenting with even heavier match bullets, particularly the 77gr Black Hills Match. The heavier bullet retains around 20% more kinetic energy in the 300-600yds range than the standard ball ammunition. Despite significantly heavier weight, due to the higher ballistic coefficient of the boat tail hollow point match design, the heavier bullets trajectory at medium ranges is very similar to the lighter bullets. At extended ranges, the 77gr is a significantly superior projectile for the .223 cartridge in all aspects, maintaining supersonic velocity some 300yds beyond the 55gr's drop to subsonic velocities. Selection of correct rate of twist is absolutely vital, with 1 in 9", or even better 1 in 9" being the only viable options to stabilize the longer heavyweight projectile. Usage of 1 in 12" with 77gr grain bullets will cause keyhole tumbling of the bullet in flight.
Law Enforcement engagement ranges are typically drastically shorter than those of the Military. Police officers utilizing patrol carbines have a huge ballistic advantage and increased hit probability over their issue sidearms. With greater range and stopping power, comes the increased risk of third parties being exposed to potential injury beyond the intended target. Whether through overpenetration or because of missed shots, the .223's use in an urban environment can be problematic at best. The fragile polycarbonate tipped 'varmint' type bullets allows the office to carry a .223 carbine with drastically reduced risk of litigation. Although the Hornady 55gr TAP round is excellent, the use of lighter varmint type bullets weighing 40grs even further reduces the likelihood of ricochet or exiting a target and subsequently causing harm to innocent citizens. Although the 55gr is stabilized satisfactorily by the 1 in 9" twist rate, the 40gr really requires the use of 1 in 12" barrels.