5.56mm
The COLT AR-15 and Its Cartridge: 223 Remington versus 5.56mm
A great deal of ongoing confusion surrounds the caliber designation of military and commercial “AR-15” series firearms. Even the term “AR-15” itself leads to a veritable minefield of miscommunication. The “AR” part of this designation does not stand for “Automatic Rifle,” “Assault Rifle,” “Atomic Reactor,” or anything else for that matter—it is simply the first two letters of “ArmaLite,” a tiny research & development company owned by Fairchild Aircraft in the 1950’s, located in Hollywood, CA and long defunct. This company worked to develop “new” firearms designs using lightweight aircraft technology—basically aluminum forgings and plastics. The ArmaLite engineers’ most famous design was the direct gas system AR-15 which was purchased by Colt’s Patent Firearms Manufacturing Company in 1959.
The select-fire AR-15 rifle, a “scaled down” derivative of the larger caliber AR-10 (308 Win), was developed around a new high velocity cartridge which propelled a small caliber bullet at over 3200 feet per second with minimum felt recoil. The cartridge was originally developed from the 222 Remington using a slightly lengthened case for increased powder volume and briefly given the designation 222 Remington Special. To avoid confusion with the 222 Remington and the longer 222 Remington Magnum, the AR-15’s cartridge was redesignated in 1959 as the now famous 223 Remington.
In the early 1960’s Colt’s 223 AR-15 rifle was tested extensively by the US Military and in 1962 the U.S. Air Force became the first branch of the armed forces to officially buy this select-fire rifle, although thousands of these original contract weapons also went to U.S. Advisors in Southeast Asia where the AR-15 and its 223 cartridge saw combat for the first time. These weapons (the original Colt, ArmaLite AR-15, Model 01) fired a 55 gr full metal jacket (FMJ) bullet from a 1/14 twist, non-chrome lined barrel. This is the not-so-quiet beginning of a weapon and ammunition controversy that exists to this day, over 50 years later.
Here are some of the highlights. The AR-15’s original 223 cartridge (222 Remington Special) used a bullet designed by Gene Stoner and Jim Sullivan at ArmaLite and manufactured by Sierra Bullet Company. When Remington began producing the 223 ammunition for the US Military, this M193 cartridge had a slightly different bullet which caused some accuracy problems and led to the adoption of the faster twist 1/12 barrel which became standard on all COLT rifles from 1964-1982. Looking more closely at the M193 round itself, we find the actual US Military specifications for a cartridge—the bullet, powder, primer, case and overall manufacturing requirements. The US cartridge M193 was given the “caliber” designations 223, 5.64mm, and later 5.56x45mm by the US Government. The term “5.56” does not actually define any “military” cartridge; it is merely the accepted metric designation for the American 223 cartridge. (Here are some others 7.62x51mm = 308 Win, 5.6mm = 22LR, 7.65mm = 32 Auto, 9mm short = 380 ACP, etc.) Interestingly enough, the AR-15/M16 has fired rounds in 222 Rem Special, 223 Rem, 5.64mm (HV), and 5.56mm with absolutely no change to its actual chamber or caliber.
The word “caliber” also creates inherent problems since it really only defines the barrel bore diameter, not the chamber specifications or specific cartridge. For example, are M1 and M14 rifles the same caliber? Yes, they both have a .30 caliber bore (.308) but they fire different cartridges 30/06 and 308 Win (7.62x51mm) respectively, which are in no way interchangeable. Most people use the word “caliber” to define a firearms cartridge which is technically incorrect.
Precisely speaking the AR-15’s .22 caliber barrel has a .224 inch groove diameter which it shares with the 22 Hornet, 22-250, 220 Swift, 222 Remington and several other non-interchangeable cartridges. The difference between all of these cartridges is the actual firing chamber dimensions and specifications. The AR-15’s firing chamber is created by its barrel, barrel extension and bolt which define the chamber size and its “headspace” or overall length. The AR-15 cartridges headspace on the barrel shoulder angle and the term “headspace” refers to the distance from the shoulder angle to the face of the bolt when the bolt is locked into the barrel extension. Like all auto-loading, military weapons, the AR-15 has a “large” chamber with special headspace dimensions which do not correspond to simple “GO,” “NO-GO” and “FIELD” gages normally used on 223 bolt action firearms. Brand new Colt AR-15/M16/M4 barrels (223/5.56mm) will often allow the bolt to close (lock) on a “NO-GO” gage, which leads us to the fact that “NO-GO” does not mean “no good” when referring to military grade Colt barrels—these barrels have special dimensions and gages.
When a cartridge is fired, the bullet leaves the firing chamber under pressure and starts into the rifling of the bore at the “leade” where it begins to rotate following the barrel’s twist rate, spinning at about 195,000 rpm from a 1/12 barrel. The leade is actually a conical relief cut into the beginning of the rifling which allows the bullet to enter the lands of the rifling with less resistance. The exact length and shape of the leade are very important because the leade has a great deal to do with accuracy and chamber pressure. The Colt AR-15 has a very specific barrel design which has changed very little since the early 1960’s. These auto-loading, military barrels have a longer chamber and leade than most 223 bolt action rifles. Today this type of barrel is often referred to as a “5.56 NATO” chamber, although NATO did not adopt a 5.56x45mm cartridge (the SS-109 bullet) until October 1980, over 25 years after the United States. Currently Colt marks most of its barrels C MP 5.56MM NATO, followed by the twist rate. The “C” means the barrel passed all Colt quality control checks and the MP certifies that the barrel passed high-pressure testing and magnetic particle inspection. Many of the shorter Colt 223/5.56mm barrels have abbreviated markings. Since 1964 all Colt select-fire lower receivers are marked with the metric “5.56MM” designation and most semi-automatic only lower receivers are marked with the American “CAL. .223,” which reinforces the fact that Colt recognizes no difference between these two “caliber” designations. Many US Military AR-15’s (Model 01 and Model 02) from the early 1960’s are marked “CAL. .223.”
As we consider ammunition more closely, we must remember one important fact—a firearm and its ammunition are an integral system, especially in auto-loading rifles and carbines. Cartridges designed for the US Government have very exacting requirements that are defined in each specific Technical Data Package (TDP). These “5.56MM” rounds include the M193 (55 gr FMJ), M196 (Tracer), M855 (62 gr FMJ using the Belgian FN SS-109 bullet), M856 (“long burning” tracer), M995 (Armor Piercing designed by Bofors in Sweden), MK 262 Mod 0 and Mod 1 (77gr open tip bullet loaded by Black Hills), and others. All of these cartridges are loaded to “military” pressures and intended only for actual military grade weapons. Commercial ammunition—whether called 223 Rem or 5.56MM—is loaded to lower pressure ratings for liability reasons and can be shot in a wide range of different firearms. This ammunition, both domestic and foreign, varies GREATLY from US Military requirements and is produced at a lower cost—often with lower quality bullets, thinner cases, higher residue (“dirty”) powder, different brass, uncrimped primers, no sealant, “looser” quality control, etc, etc. These rounds can also vary greatly from one production lot to another. Needless to say, there are ongoing “issues” with “lower quality ammo” damaging COLT Law Enforcement weapons.
No matter what we call the various COLT AR-15 rifles and carbines, or what we call the “caliber” from one day to the next, these classic direct-gas, military-grade firearms are very dependent on HIGH quality, military-grade ammunition—by design.
KE