5.56x45mm NATO

01 / At a glance

The basics

Origin

Belgium / NATO

Introduced

1980

Ignition

Centerfire

Case form

Rimless, bottleneck

Diameter

5.7 mm

Case length

44.7 mm

Rounded values to help you identify the cartridge, not manufacturing drawings. See the cited standard for exact dimensions and tolerances.

A short history

The 5.56 NATO descends from the .223 Remington, which Remington designed in the early 1960s for the AR-15 rifle. The military version of that rifle became the M16, and the cartridge became the standard round for American infantry.

NATO chose a second standard rifle cartridge after the 7.62x51mm NATO. FN Herstal in Belgium developed a new bullet and load in the late 1970s, and NATO tested candidates from 1977 to 1980. On 28 October 1980 it standardized the SS109 round under STANAG 4172. The United States calls its version the M855.

The SS109 bullet weighs 62 grains and has a steel penetrator in the nose. It is heavier than the earlier 55 grain M193 bullet, and it needs a faster rifling twist to stay stable. That change is the root of many of the differences between military and civilian ammunition and chambers.

Today the cartridge is used by NATO members and many other countries. In the United States the same case is sold to civilians in huge quantities, labeled either .223 Remington or 5.56x45mm NATO.

Why it is so common

The first reason is the rifle. The AR-15 pattern is the best-selling rifle design in the United States, and almost all of them are chambered in .223 Remington, 5.56 NATO or a hybrid chamber. That created a huge market for parts, accessories and training.

The second reason is recoil. A 5.56 rifle kicks very little. A new shooter can fire a long session without fatigue, and follow-up shots are fast. For training, that is a real advantage.

The third reason is supply. The cartridge is made in military and commercial plants around the world, and it is sold in nearly every store that sells ammunition. It is one of the most widely sold centerfire rifle cartridges in the country.

Specifications at a glance

Pressure figures from different standards use different measurement methods, so they cannot be compared directly. Exact dimensions and tolerances are in the standards cited at the end of this page.

5.56 NATO and .223 Remington

The two cartridges have the same case and bullet dimensions. The difference is mainly in the chamber and the pressure rating. A 5.56 NATO chamber has a longer leade, which is the distance from the case mouth to where the rifling starts. A .223 Remington chamber can have a shorter leade.

Because of this, 5.56 NATO ammunition should not be fired in a rifle chambered only for .223 Remington. The longer leade of the NATO chamber lowers peak pressure, and a short leade does not. A .223 Remington cartridge can be fired in a 5.56 NATO chamber, though accuracy may be a little lower.

Some rifles have a hybrid chamber, such as the .223 Wylde, that accepts both. If a rifle is marked 5.56 NATO, it is made for both. If it is marked only .223 Remington, use .223 ammunition. When in doubt, read the barrel marking and the manual.

Strengths and limits

The 5.56 NATO is a light, fast cartridge. It is excellent at what it is made for and limited outside that.

It is a very good cartridge for varmints, predators and target work. It can be used on small deer in places where the law allows, but a larger cartridge is a better choice for deer. Always check your state rules before you hunt with it.

Barrel length and velocity

Barrel length changes velocity. A 20 inch barrel gives the speeds that ammunition makers publish. A 16 inch carbine gives somewhat less, and a very short barrel gives much less. The loss also changes how a bullet performs at long range.

Ballistics

The tables and charts below show one common factory load, a 55 grain FMJ boat-tail bullet at 3,165 ft/s, with the rifle zeroed at 100 yards. A 100-yard zero is common for AR-15 pattern rifles.

Reading the numbers

With a 100-yard zero the bullet falls about 3 inches at 200 yards, 12 at 300, 29 at 400 and 58 at 500. A 10 mph crosswind moves it about 5 inches at 200 yards, 12 at 300, 23 at 400 and 38 at 500. Velocity drops from 3,165 ft/s at the muzzle to about 1,500 ft/s at 500 yards, and energy from about 1,220 to about 280 foot-pounds.

The light bullet sheds speed quickly. Inside 300 yards the cartridge is accurate and flat, and past that the drop and wind drift grow fast. Heavier, higher-ballistic-coefficient bullets of 69 to 77 grains do better at long range, and they need a faster twist.

Recoil and shooter comfort

Recoil is very light. A 6 to 8 pound AR-15 pattern rifle pushes back gently, and the muzzle rises little. Children and new shooters can learn on it, and adults can shoot it all day. The report is sharp in a short barrel, so hearing protection matters.

Light recoil lets a shooter see the bullet strike and fire again quickly. That is why the cartridge is used so widely for training and for competition that rewards speed.

How to choose ammunition

Choose by purpose first, then by weight. Pay attention to the marking on the box and the rifle chamber.

Check the twist

A slow twist, such as 1 in 12, stabilizes light bullets of 55 grains or less. A 1 in 9 twist stabilizes bullets up to about 65 grains. A fast twist, such as 1 in 7, stabilizes 77 grain bullets and long match bullets. Choose a bullet weight that suits your barrel.

Steel-case and range rules

Some ranges ban steel-core and steel-case ammunition, because steel can start fires and damage targets. Check the range rules before you buy ammunition in bulk.

How to choose a rifle

The right rifle depends on how you will use it. These are the points that matter most.

• Chambering. A rifle marked 5.56 NATO or .223 Wylde is the safe choice. Do not buy a rifle chambered only for .223 Remington if you plan to shoot 5.56 ammunition.
• Barrel length. 16 inches is the usual carbine length and handles well. 18 to 20 inches gives more velocity. A longer barrel suits target work.
• Twist rate. A 1 in 7 or 1 in 8 twist handles the widest range of bullets. A 1 in 12 twist suits only light bullets.
• Weight. A light rifle of 6 to 7 pounds is easy to carry. A heavier rifle is steadier on the bench.
• Optic. A red dot or a low-power variable scope of 1-6x suits most uses. A magnified scope of 4-16x suits varmint and target work.
• Quality. An AR-15 is a platform with many makers. Choose a maker with a good record and keep to standard parts.

Zeroing and practice

A 100-yard zero is the most common and the easiest to check. A 50 and 200 yard zero is popular on carbines. It puts the bullet on target at both distances, with only a small rise in between. Choose one and learn your own numbers.

Practice from field positions, and shoot at the distances you expect to use. A rifle that groups well at the bench but is never fired from a sling or a rest gives false confidence. Confirm your zero with the ammunition you plan to use, and check it again when you change loads.

Practical uses

Target shooting and training

This is the main use. The light recoil and low cost make it a good choice for practice, and it is used in many competitions.

Varmints and predators

With a soft point or polymer-tip bullet it is effective on prairie dogs, coyotes and fox to a few hundred yards. Matching the bullet to the twist matters for accuracy.

Home defense

Many people keep an AR-15 for home defense. The light recoil allows fast, accurate shooting, but the bullet can pass through walls, so the choice of load and the surroundings matter.

Small deer and hogs

It is used for these where the law allows it, with a bullet made for hunting and a careful shot. A larger cartridge is a better choice.

Plinking

It is fun to shoot, but ammunition costs more than for rimfire cartridges.

5.56 NATO vs similar cartridges

The table lists the cartridges most often compared with the 5.56 NATO. Figures are rounded, and pressures are from the standards named in each row.

Against the .223 Remington, the case is the same, and the chamber and pressure rating differ. Against the .300 AAC Blackout, it is faster and flatter with a lighter bullet. Against the 7.62x39, it has a flatter path and less recoil, and a lighter bullet. Against the .22-250 Remington, it is slower and kicks less.

Care and storage

Clean the bore, the chamber and the bolt carrier group after each session. A gas-operated rifle collects carbon in the action, and a light oil on the moving parts keeps it running. Check the screws and the optic mounts from time to time. Store the rifle unloaded in a dry place. Keep firearms and ammunition locked and out of reach of children and unauthorized users.

Safety and common mistakes

• Do not fire 5.56 NATO ammunition in a rifle chambered only for .223 Remington. Read the marking on the barrel.
• Treat every firearm as loaded. Keep your finger off the trigger until your sights are on the target.
• Know your target and what is beyond it. A rifle bullet can travel more than a mile.
• Check that the bore is clear before shooting.
• Wear eye and ear protection at the range.
• Follow range rules on steel-core and steel-case ammunition.
• If you reload, use only published data from a current manual and start low. This page does not contain loading data.
• Store firearms unloaded and locked, and keep the ammunition apart.

Sources

Sources reviewed · 29 October 2026

1. Wikipedia · 5.56×45mm NATO ↗
2. Federal · American Eagle 5.56x45mm 55 gr FMJ BT ↗
3. SAAMI ↗