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Difference Between 5.56 and 5.56 NATO

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The 5.56 and 5.56 NATO are two cartridges that, at first glance, may seem identical. However, it's crucial to understand the differences between these two popular rounds as they can significantly impact both firearm performance and safety.

The main difference between 5.56 and 5.56 NATO is that they have distinct chamber pressures and dimensions, which can affect compatibility and safety when used interchangeably.

In this article, we will delve into the historical context surrounding the development of both cartridges, discuss their technical specifications, and examine the key differences between them.

Comparison Table: 5.56 Vs 5.56 NATO

Feature5.565.56 NATO
Chamber Pressure55,000 psi (approx.)62,000 psi (approx.)
Chamber DimensionsShorter leadeLonger leade
Compatibility5.56 firearms5.56 NATO firearms
Interchangeability RisksSafe in 5.56 NATOUnsafe in 5.56
Bullet Weight (typical)55-62 grains62-77 grains
Muzzle Velocity (typical)2,900-3,200 fps2,750-3,150 fps
Muzzle Energy (typical)1,100-1,300 ft-lbs1,000-1,300 ft-lbs
Common Bullet TypesFMJ, SP, HPM855, M856, MK 262
Practical ApplicationsCivilian, Law EnforcementMilitary, Law Enforcement, Some Civilian

Please note that the values for muzzle velocity and muzzle energy may vary depending on specific load and barrel length. The table serves as a general comparison between the two cartridges.

Historical Context

Development of 5.56

M16 1
The M16 assault rifles originally used the 5.56×45mm ammunition before changing to the 5.56mm NATO

The 5.56 cartridge originated in the late 1950s as a result of the U.S. military's search for a lighter, smaller-caliber round to replace the 7.62x51mm NATO cartridge. The goal was to develop a round that offered reduced weight and improved ballistic performance, allowing soldiers to carry more ammunition and enhance their effectiveness on the battlefield. ArmaLite, a small American arms company, developed the .223 Remington cartridge, which was eventually adapted and designated as the 5.56x45mm by the military.

The 5.56 cartridge was first adopted by the U.S. military in 1962, with the introduction of the M16 rifle. Since then, the cartridge has evolved over time, with various bullet designs and powder loads being developed to improve performance and meet specific military requirements.

Development of 5.56 NATO

hk 416
The HK416 uses the 5.56 mm NATO ammunition

The formation of the North Atlantic Treaty Organization (NATO) in 1949 aimed to promote cooperation and standardization among member countries in various aspects, including military equipment and ammunition. The adoption of the 7.62x51mm NATO cartridge in the 1950s was the first step towards ammunition standardization. However, as the U.S. military began to adopt the 5.56 cartridge, the need for a new standardized cartridge became apparent.

In 1980, NATO officially adopted the 5.56x45mm NATO cartridge, which was based on the U.S. military's 5.56 cartridge but with specific modifications to meet NATO requirements. The 5.56 NATO cartridge was designed to ensure interoperability among firearms and ammunition produced by NATO member countries, allowing for efficient logistics and increased compatibility on the battlefield.

Since its adoption, the 5.56 NATO cartridge has become a standard for military forces across the globe, with various firearms being designed or adapted to chamber the round, such as the M4 Carbine, FN SCAR, and HK416.

Technical Specifications


The 5.56 cartridge, originally designed as the .223 Remington, features a bullet diameter of 5.56 mm (0.219 inches) and typically weighs between 55 and 62 grains, depending on the specific bullet design. The cartridge has a case length of 44.70 mm (1.760 inches) and a rim diameter of 9.60 mm (0.378 inches).

Muzzle velocities for the 5.56 cartridge can range from 2,900 to 3,200 feet per second, depending on the specific load and barrel length, resulting in muzzle energies of approximately 1,100 to 1,300 foot-pounds.

Common bullet types for the 5.56 cartridge include:

  • Full metal jacket (FMJ): A bullet with a soft core, usually made of lead, encased in a harder metal such as copper or steel.
  • Soft point (SP): A bullet with a soft lead core and a partially exposed tip, designed to expand upon impact for increased terminal performance.
  • Hollow point (HP): A bullet with a hollow cavity in the tip, intended to expand upon impact for enhanced terminal performance and reduced risk of over-penetration.

5.56 NATO

The 5.56 NATO cartridge shares the same bullet diameter of 5.56 mm (0.219 inches) as the 5.56 cartridge, with bullet weights generally ranging from 62 to 77 grains, depending on the specific load. The 5.56 NATO has a case length of 44.70 mm (1.760 inches) and a rim diameter of 9.60 mm (0.378 inches), just like the 5.56 cartridge.

Muzzle velocities for the 5.56 NATO cartridge can range from 2,750 to 3,150 feet per second, depending on the specific load and barrel length, resulting in muzzle energies of approximately 1,000 to 1,300 foot-pounds.

NATO standardized bullet types for the 5.56 NATO cartridge include:

  • M855 (SS109): A 62-grain, full metal jacket bullet with a steel penetrator core, designed for improved penetration against light armor and barriers.
  • M856 tracer: A 63.7-grain bullet with a tracer element, allowing for visible tracking of the projectile's flight path, primarily used for machine gun fire adjustment.
  • MK 262 Mod 1: A 77-grain, open-tip match bullet with improved accuracy and terminal performance, designed for use in designated marksman and sniper roles.

Key Differences

Chamber Pressure

One of the main differences between the 5.56 and 5.56 NATO cartridges is their respective chamber pressures. The 5.56 NATO cartridge typically has a higher chamber pressure compared to the 5.56 cartridge. While the 5.56 cartridge has a maximum average pressure of approximately 55,000 psi (pounds per square inch), the 5.56 NATO cartridge has a maximum average pressure of approximately 62,000 psi.

This difference in chamber pressure has implications for firearm performance and safety. Firearms designed for 5.56 NATO typically have stronger barrels and receivers to withstand the increased pressure, while firearms designed for 5.56 may not be able to safely handle the higher chamber pressure of 5.56 NATO ammunition.

Chamber Dimensions

In addition to chamber pressure, there are differences in chamber dimensions between firearms designed for 5.56 and 5.56 NATO cartridges. The 5.56 NATO chamber has a longer throat, or leade, compared to the 5.56 chamber. The leade is the portion of the barrel directly in front of the chamber where the rifling begins.

This difference in leade length can impact accuracy and create potential risks when using the wrong type of ammunition. Using 5.56 NATO ammunition in a firearm with a 5.56 chamber may result in increased pressure, as the bullet engages the rifling sooner. This can lead to excessive wear, decreased accuracy, and in extreme cases, catastrophic failure of the firearm.

Interchangeability and Compatibility

Given the differences in chamber pressure and dimensions, it is crucial to understand the potential dangers of using 5.56 NATO and 5.56 cartridges interchangeably. Using 5.56 NATO ammunition in a firearm designed for 5.56 can result in excessive pressure, risking damage to the firearm and injury to the shooter. Conversely, using 5.56 ammunition in a firearm designed for 5.56 NATO is generally considered safe, although it may result in slightly decreased accuracy due to the longer leade in the 5.56 NATO chamber.

To ensure firearm safety and optimal performance, it is best to follow the manufacturer's recommendations and use the appropriate ammunition for your specific firearm. Always check the markings on your firearm to determine whether it is chambered for 5.56, 5.56 NATO, or both.


In conclusion, recognizing the critical differences between the 5.56 and 5.56 NATO cartridges is essential for firearm performance, safety, and proper application. By comprehending their distinct specifications and historical development, users can make informed decisions regarding suitable ammunition for their specific needs. Whether for military, law enforcement, or civilian use, understanding the nuances between these two cartridges ensures safe and effective firearm operation.

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About the Author: Nicolas Seignette

Nicolas Seignette, who holds a scientific baccalaureate, began his studies in mathematics and computer science applied to human and social sciences (MIASHS). He then continued his university studies with a DEUST WMI (Webmaster and Internet professions) at the University of Limoges before finishing his course with a professional license specialized in the IT professions. On 10Differences, he is in charge of the research and the writing of the articles concerning technology, sciences and mathematics.
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