Gregory Kielma • October 21, 2023
223 Remington/5.56 x 45mm NATO Cartridge
223 Remington/5.56 x 45mm NATO Cartridge
Hey Gregory,
Today's workbench is written by guest contributor and AGI Certified Gunsmith and GCA Moderator Robert Dunn:
There isn’t a day of my life that goes by that I don’t work with, reload, think or write about the .223 Remington and 5.56 NATO cartridges. Those cartridges are what a couple of my favorite and most used rifles are chambered for.
When I worked in an ammunition factory, I would sometimes get paid in .223 ammunition. When I was growing up in the early 1970s I became familiar with the .223 Remington cartridge, as it was the new round being used by our Armed Forces.
Warfighters throughout the world had realized the advantage of automatic firepower, but finding the perfect firearm and cartridge design is a huge challenge. In the case of the .223 Remington, many sources helped bring this cartridge into use by both civilians and the military.
Eugene Stoner of ArmaLite (then a division of Fairchild Industries) had gained the attention of the Military with his 7.62 x 51mm chambered AR-10 rifle. Like many high powered rifles, the AR-10 was hard to control during automatic fire.
Consequently, Fairchild Industries and Stoner were asked to scale down the AR-10 to fire a smaller caliber cartridge. Stoner came up with a prototype AR-15 and after a demonstration for the army, he was asked to produce more rifles for testing in 1957.
Select engineers and other companies like Remington Arms, Winchester as well as ArmaLite were asked to take part in developing this project. Springfield Armory’s Earle Harvey lengthened the .222 Remington case to the required specifications and it became the .224 Springfield, which was commercially known as the .222 Remington Magnum.
Frank Snow (of Sierra Bullets) and Eugene Stoner calculated the ballistics data for the .222 Remington to find the best bullet for the project, which ended up being a 55 grain bullet. Meanwhile, the Technical Editor for Gun and Ammo magazine, Robert Hutton, worked up a propellant recipe that could attain a velocity of 3,300 fps. Ultimately, Remington came up with the .222 Special cartridge, which was later renamed the .223 Remington because there were so many .222 designations.
Just a few of the military requirements for the firearm and cartridges were that they needed to be able to penetrate one side of a US Steel helmet at 500 yards, had to be able to exceed supersonic speed at 500 yards, had to be .22 caliber and select fire. From years of research and development of the AR-10 and AR-15, ArmaLite was facing financial difficulties and sold the rights for the rifles to Colt in 1959.
The AR-15 and the T44E4 rifle that would later be known as the M14 were being testing at the same time. To make a long story short, it was found that the AR-15 had far less failures per 1,000 rounds fired and that riflemen had a higher hit probability when firing Stoner’s rifle.
After Air Force General Curtis Le May test fired the AR-15, he ordered a batch of AR-15s to replace some of the M2 carbines that the Air Force was using. As the trials and testing continued, it was found that during marksmanship testing that many more men were achieving Expert shooting status with the AR-15 than they were with the M14. With these findings and the failure rate of the AR-15 going down even further, General Le May placed an order for 80,000 rifles in 1961.
In 1962, Remington filed the specs for the .223 Remington cartridge with SAAMI (Sporting Arms and Ammunition Manufacturer’s Institute) and the cartridge was later marketed commercially as a chambering for the Remington 760 rifle in 1963.
At this point in time, the United States was embroiled in the conflicts arising in Southeast Asia. It was politically decided that contracts for rifle manufacturing could be bid on by the private sector and Colt was awarded the contract to provide AR-15 rifles to fill orders for the Vietnam War.
The AR-15 rifle and the .223 Remington cartridge were formally adopted in 1964 and designated the XM16E1 rifle, commonly referred to as the M16, and the 5.56 Ball M193 cartridge. It was decided that the South Vietnamese troops would be supplied with the M16 rifle. Eventually, the Air Force, the US Army, Special Forces, the Coast Guard, the US Navy, the Marines as well as the SEAL teams had orders in for M16 rifles and cartridges. In 1965, Colt received orders for 800,000 M16E1 rifle and over 28,000 M16 rifles.
The M16 and the 5.56 Ball ammo would go through some major growing pains as the rifles and cartridges were issued to troops in Vietnam. Besides not being trained on how to properly clean the gun, the rifle suffered from many types of jams and failures.
Broken extractors and extractor springs accounted for many ejection failures and through more research, it was found that the propellant being used in the cartridges was creating horrific fouling of the gas and bolt system. The ball powder that was being used also created a longer pressure spike, which caused the case to expand in the chamber but it would not contract enough to be extracted.
The extractor would simply tear through the case rim, leaving the stuck case in the chamber. By using a different propellant, chrome plating the barrels and replacing the buffer with a new design that slowed down the cyclic rate, along with some other design changes, the problems were gradually sorted out. Unfortunately many American lives were lost because of these errors.
The process of selecting a smaller cartridge for use by NATO forces began in 1960. The cartridge design was based on the .223 Remington cartridge and its development was placed in the hands of FN Herstal. The rimless bottlenecked cartridge was named the 5.56 x 54mm NATO cartridge.
In 1980, the 5.56 NATO cartridge became the standard NATO rifle cartridge (known as the SS109 for NATO and the M855 for the United States). The 5.56mm cartridges allowed the soldier to be able to carry more ammunition, which equated to more firepower and logistically many more rounds could be transported to hot spots around the world by weight than the larger 7.62 NATO cartridges. On a global scale this can equal a savings of millions of dollars.
The number of firearms that are chambered for the 5.56 NATO cartridge is too numerous to list here. Just think about all of the different squad automatic weapons, rifles, carbines and machine guns that each NATO country uses…that is a lot of different weapons!
The Frankford Arsenal, Remington and Winchester produced the first lots of .223 ammunition for the United States Armed Forces. During the trials and testing of the AR-15, 19 Million .223 cartridges were purchased. When a cartridge does well for the military, the Law Enforcement and civilian communities usually follows their lead. If you have ever read the 27 page document for the military specifications for the M855 cartridge (5.56 x 54mm), you can see that heavy scrutiny has already been applied to the 5.56mm round!
The .223 Remington cartridge has become one of the most used cartridges in the United States. It is a great varmint and predator cartridge and could be used for larger game if need be by loading a heavier bullet like a Hornady GMX or Barnes TSX 70-grain projectiles. These slower moving heavier bullets seem to do quite well in windy conditions. The above attributes make the 5.56mm round a flexible survival cartridge too.
You may have noticed or even been annoyed by the interchanging .223/5.56 terminology, here are of the important similarities and difference between the two cartridges. The case dimensions are exactly the same, though case thickness/propellant capacity can vary significantly between manufacturers which will have an effect on the resulting pressures.
The 5.56 x 45mm NATO or other military 5.56 ammo tends to be loaded hotter, so chamber pressures will be greater than the typical .223 Remington load. For this reason, .223 Remington rounds can be safely fired in a 5.56 chambered gun but 5.56mm ammo should not be fired in a .223 Rem chambered gun.
The reason for this really comes down to the differences in the chamber dimensions between .223 and 5.56 barrels. The throat/free bore and leade of a 5.56mm chamber is longer. The throat of the chamber is the unrifled section of the chamber just forward of the neck and gives the bullet a place to sit. Leade or lead is the section of the bore of a rifled barrel located just ahead of the throat of the chamber. It is a conical shaped transition from end of the throat to the fully rifled part of the barrel.
In an effort to gain more accuracy from the 5.56mm NATO cartridge, Bill Wylde created the .223 Wylde, which is a chamber design that utilizes the same external dimensions and lead angle of a 5.56 NATO cartridge and the freebore diameter of the .223 Remington cartridge. Another company that uses a proprietary chamber design to achieve better accuracy from the 5.56mm NATO cartridge is Noveske. A Noveske barrel is not cheap but they are known for their precision and high quality products.
SAAMI specs are followed by manufacturers in the U.S. loading .223 Remington ammo but those same manufacturers who also load 5.56 ammo do not follow SAAMI specs because the 5.56 rounds are loaded to military specifications and thus not held to SAAMI specs. In Europe, the international organization C.I.P. tests and sets the safety standards for firearms and ammunition. In countries that are regulated by the C.I.P., .223 Remington ammunition is proof tested at the same maximum service pressure as the 5.56 x 54mm NATO cartridges.
There have been a lot of challenges that had to be met throughout the history of the AR-15 and the 5.56 x 54mm cartridge. The twist rate of a barrel and the chamber dimensions greatly effect performance and accuracy. The shorter barreled M4 carbines and the requests for even shorter barrel lengths coming from the Special Forces communities have kept firearms and ammunition engineers busy. At this point in time, the AR-15 style rifles and the .223 Rem and 5.56 NATO cartridges offer a very reliable and accurate shooting platform.
The AR-15 rifle and .223/5.56 cartridge combination have gone through many changes together. They are still being used by our Military/Law Enforcement and civilian populations to good affect every day. This combo is a must for 3-gun competition. The .223 Remington cartridge is not overly expensive and there is always a wide variety of ammunition to choose from. There is a lot of surplus ammo to select from as well. Many other firearms are chambered in .223 Rem besides the AR-15 type guns.
The 5.56mm round is a good cartridge to reload for, as there is a huge variety of quality components to choose from as well. In 1964, the year I was born, these cartridges were the new warfighting cartridge and the US Armed Forces began using them. It’s easy for me to remember how many years these cartridges have been in use!
Become an American Gunsmith and continue to preserve our Firearm’s Heritage!
Robert Dunn,
Gunsmith & Community Moderator
American Gunsmithing Institute
Gunsmithing Club of America
Convicted Felon Sentenced to 87 Months in Trafficking Nine Firearms, Including to Buyer Who Said He Was ‘At War’ Thursday, April 30, 2026 U.S. Attorney's Office, District of Columbia WASHINGTON - Brandon Smith, 34, a previously convicted felon residing in the District of Columbia, was sentenced today in U.S. District Court to 87 months in prison for conspiring to traffic at least nine firearms to a prohibited buyer over the course of six months, announced U.S. Attorney Jeanine Ferris Pirro. “Brandon Smith was already on supervised probation for a violent felony when he chose to traffic firearms, and he continued even after being told the buyer intended to use them for violence,” said U.S. Attorney Pirro. “Over the course of six months, he arranged the sale of at least nine guns—including one with an obliterated serial number—to a prohibited individual. This was not a momentary lapse in judgment, but a sustained and deliberate effort to arm someone who could not legally possess firearms. My office remains committed to holding accountable those who endanger our communities by trafficking illegal guns.” On Jan. 9, 2026, Smith pleaded guilty before Judge Howell to conspiracy to commit trafficking in firearms. In addition to the 87-month prison term, Judge Howell ordered Smith to serve three years of supervised release. Federal prosecutors had requested a 108-month prison term. According to court papers, beginning in November 2023, ATF opened an investigation after a confidential source reported that Smith, then on supervised probation for a violent felony, was actively advertising firearms for sale by texting photographs of guns to prospective buyers, including individuals with prior felony convictions. During the next six months, Smith sold or arranged the sale of nine firearms to a buyer on six separate occasions. During the transactions, Smith sold his own personal carry firearm on multiple occasions when a supplier failed to deliver, then purchased a replacement for himself afterward. In early January 2024, as Smith and the buyer discussed an upcoming transaction, the buyer told Smith he needed the firearms because he was “at war” after his cousin had been killed. Smith proceeded with the sale. The buyer had also told Smith he was serving a criminal justice sentence at the time of the transactions. Smith acknowledged that he, too, was “on papers.” Smith arranged a total of six transactions from Nov. 30, 2023, through May 30, 2024, resulting in the sale of nine firearms. At least one of the firearms had its serial number obliterated. On Oct. 26, 2024, MPD officers conducted a traffic stop on the 1600 block of 16th Street SE and found Smith in the front passenger seat of a parked vehicle. Officers observed open containers of alcohol and discovered a satchel at his feet. Inside the satchel, in plain view, was a loaded Glock Model 19X 9mm handgun with a round in the chamber and 16 additional rounds in the magazine. The bag also contained a bank card and government-issued identification in Smith’s name. Smith has prior convictions for Simple Assault (2011), Attempted Robbery (2013), and Robbery and Possession of a Firearm during a Crime of Violence (2016), for which he was sentenced to five years in prison. He was serving a term of supervised probation from the 2016 conviction at the time of the firearms trafficking conspiracy. This investigation was conducted by the Bureau of Alcohol, Tobacco, Firearms and Explosives Washington Field Office, and the Metropolitan Police Department. The matter was prosecuted by Special Assistant U.S. Attorney Brendan M. Horan. Convicted Felon Sentenced to 87 Months in Trafficking
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