The small-caliber, high-velocity idea
After World War II, the U.S. Army studied how soldiers actually fought and wondered whether a lighter rifle with lighter ammunition would serve better than the heavy .30-caliber rifles of the day. The result was the SCHV project, short for small-caliber, high-velocity.
According to Wikipedia, early work began in 1957. The Army asked Eugene Stoner of ArmaLite to scale down his 7.62mm AR-10 design. In May 1957, Stoner demonstrated a prototype AR-15 in live fire for General Willard Wyman.
The rifle needed a cartridge that could push a 55-grain bullet at around 3,300 feet per second. The starting point was the .222 Remington, a popular varmint round.
From .222 Special to .223 Remington
Several people worked on the case. Springfield Armory's Earle Harvey lengthened the .222 Remington case, and Stoner and Frank Snow worked on their own version. Remington then created a larger cartridge called the ".222 Special."
In 1959 the .222 Special was renamed .223 Remington, which avoided confusion with the existing .222 family. Remington submitted its specifications to SAAMI, the U.S. industry standards body, in 1962.
Meanwhile the rifle was proving itself. In 1958 reliability testing, the AR-15 recorded fewer failures per thousand rounds than the rifle that became the M14. Air Force General Curtis LeMay ordered AR-15s, and in 1961 he ordered 80,000 rifles.
The M193 and the M16
In September 1963, the cartridge was officially accepted as "Cartridge, 5.56mm ball, M193." It fired a 55-grain bullet at about 3,250 feet per second, according to Wikipedia.
The rifle became the M16. Its early service in Vietnam was troubled, and the reasons are still debated. But the cartridge stuck, and the M193 became one of the most widely produced rifle loads in the world. Today, M193-pattern 55-grain FMJ is a staple of the bulk ammo market. Our M193 vs M855 comparison explains how it differs from its successor.
NATO's 1980 decision and the SS109
In 1970, NATO members agreed to select a smaller cartridge to sit alongside the 7.62x51mm. FN Herstal of Belgium developed a new design based on the .223 Remington case with a heavier bullet. NATO designated it SS109; the U.S. called it M855.
On October 28, 1980, under STANAG 4172, the cartridge was standardized as NATO's second standard service rifle cartridge. The SS109 used a 62-grain bullet with a steel penetrator and a lead core. The U.S. first fielded M855 with the M16A2. In 2010, the U.S. Army adopted the M855A1 to replace it.
Much of that military ammunition has been made at the Lake City Army Ammunition Plant. Our Lake City history covers why "LC" headstamps are so common on bulk 5.56.
Why .223 and 5.56 are not quite the same
The two cartridges share nearly identical external dimensions, but chamber and pressure specifications differ:
| .223 Remington | 5.56x45mm NATO | |
|---|---|---|
| Standards body | SAAMI (U.S. industry) | NATO / CIP |
| Chamber leade | Shorter | Longer ("NATO" or "mil-spec" chamber) |
| Pressure | Measured and rated by SAAMI method | Rated by NATO / CIP methods; generally loaded hotter |
Wikipedia notes that firing 5.56 NATO in a rifle chambered only for .223 Remington can raise pressures to unsafe levels, and SAAMI advises against it. Firing .223 in a 5.56 chamber is generally considered safe. Always go by your barrel's marking and your manufacturer's guidance.
What this means for bulk buyers
- Know your chamber. A barrel marked 5.56 NATO or .223 Wylde can generally handle both. One marked only .223 Remington should get .223 ammo.
- Read labels carefully. Some listings describe .223 ammo with "5.56" in the title, or vice versa. Check the actual cartridge on the box.
- Choose the bullet weight for your barrel. Older barrels with slow twist rates may stabilize 55-grain better than 62-grain.
- Green tips are M855 or SS109-type. Some ranges restrict steel-penetrator ammo. Check before you buy a case.
How the history shows up on today's bulk listings
Almost every phrase you see on a 5.56 or .223 listing traces back to this history. Knowing the vocabulary helps you buy the right case.
| Listing term | What it usually means |
|---|---|
| M193 or XM193 | 55-grain FMJ loaded to the M193 pattern; the classic bulk AR load |
| M855, SS109 or "green tip" | 62-grain bullet with a steel penetrator |
| .223 Remington | Loaded to SAAMI .223 specifications |
| 5.56x45mm or 5.56 NATO | Loaded to military-style specifications |
| LC headstamp | Cases made at the Lake City Army Ammunition Plant |
| Lake City or "LC overrun" | Commercial ammo made at Lake City, often sold under a manufacturer's brand |
The "X" in XM193 is worth a note. In military designations, an X prefix traditionally marks an experimental or developmental item. On commercial listings, XM193 generally describes ammo made to M193-style specifications for the civilian market, but each maker uses the term its own way. Read the product description rather than relying on the designation alone.
Bullet weight and twist rate
The jump from 55-grain M193 to 62-grain SS109 came with a change in rifle barrels. Heavier, longer bullets need a faster rifling twist to stabilize. Early M16 barrels used slower twists suited to the 55-grain bullet; the M16A2 moved to a faster twist for the 62-grain M855.
That still matters to bulk buyers. Most modern AR-15 barrels use twists that handle both 55- and 62-grain bullets, but some older or budget rifles with slow twists may struggle with heavier bullets. If your groups open up or you see oblong holes in the target, the bullet may not be stabilizing. Check your barrel's twist rate, which is often stamped on the barrel or listed in the manual, before buying a case of heavier ammo.
The safest approach is the same as for any caliber: buy a few boxes of the loads you are considering, shoot them in your rifle, and buy the case of the one that groups and functions best.
Storing a case of 5.56
A 1,000-round case of 5.56 or .223 is heavier and bulkier than the same count of 9mm, but it stores the same way: in sealed, gasketed cans with desiccant, cool and dry, off the floor. Ammo packed in battle packs or spam-can-style sealed packaging can stay sealed until you need it. Label everything with the cartridge name, bullet weight and lot, especially if you keep both .223 and 5.56 in the house.
Frequently asked questions
When was .223 Remington introduced?
The cartridge was renamed .223 Remington in 1959 and its specifications were submitted to SAAMI in 1962. The Army accepted it as the 5.56mm M193 in 1963.
Who designed the AR-15?
Eugene Stoner of ArmaLite developed the AR-15 by scaling down his AR-10 design in the late 1950s.
When did NATO adopt 5.56?
NATO standardized the 5.56x45mm, based on FN's SS109, on October 28, 1980 under STANAG 4172.
Can I shoot 5.56 in a .223 rifle?
SAAMI advises against firing 5.56 NATO in a chamber marked only .223 Remington. Follow your barrel marking and manufacturer guidance.
What is the difference between M193 and M855?
M193 uses a 55-grain FMJ bullet. M855 uses a 62-grain bullet with a steel penetrator, usually marked with a green tip.
Editorial note: This history draws on Wikipedia's 5.56x45mm NATO entry and other published references. We have not tested products hands-on. Follow your firearm manufacturer's guidance on ammunition. Prices and stock change. We may earn a commission if you buy through our links.
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