What is 5.56 NATO? It is the 5.56x45mm military rifle cartridge, a .224-inch bullet in a 45 mm bottleneck case, adopted by NATO as its standard small-caliber round and fired by nearly every AR15. Most shooters shorten it to 5.56 or 556. A 5.56 NATO chamber also fires .223 Remington, but a .223 Remington chamber should not fire 5.56 ammunition. Chattahoochee Munitions manufactures complete 5.56 NATO firearms on its CM15 AR15 platform: 16-inch rifles, 10.5-inch and 8-inch pistols or SBRs, and 8, 10.5 and 16-inch complete uppers.

What Is 5.56 NATO in Plain Terms?

5.56 NATO is a small, fast, light-recoiling rifle cartridge. Typical factory bullets weigh 55 to 77 grains and leave a 16 to 20-inch barrel at roughly 2,700 to 3,200 feet per second. The result is a flat trajectory to about 300 yards, mild recoil, light ammunition and magazines that hold 30 rounds in the space a larger cartridge needs for 20. Those traits are why the 5.56 NATO became the default AR15 chambering for training, defense, competition and varmint shooting.

5.56 NATO Specifications

Official names: 5.56x45mm NATO, 5.56 NATO; commonly written 5.56, 556 or 5.56×45
Parent cartridge: .223 Remington (same external case dimensions)
Bullet diameter: .224 inch
Case length: 1.760 inches (44.70 mm)
Maximum overall length: 2.260 inches (57.40 mm)
Bolt face: .378 inch, the standard AR15 bolt
Common bullet weights: 55, 62, 69, 75 and 77 gr
Customary twist rates: 1:7 or 1:8 (Chattahoochee Munitions uses 1:7)
Platform: AR15 (standard lower receiver, bolt and magazine)

The .223 Remington is standardized by SAAMI. In October 2026 SAAMI also accepted a voluntary commercial standard, the 5.56 COMM, with a 62,000 psi maximum average pressure against 55,000 psi for .223 Remington, as American Rifleman reported. The military 5.56 NATO itself is defined by NATO and military specifications rather than a public SAAMI drawing.

Is 556 the Same as 5.56 NATO?

Yes. “556”, “5.56”, “5.56×45” and “5.56 NATO” all name the same cartridge. The short forms are just how people type it. Chattahoochee Munitions keeps the official name, 5.56 NATO, on its product pages and names the short forms so a search for any of them finds the same answer.

What Is 5.56 NATO Compared With .223 Remington?

The two cartridges share external case dimensions, so they look identical. The difference is the chamber and the pressure. A 5.56 NATO chamber has a longer leade (the throat ahead of the case mouth) and 5.56 ammunition is loaded to higher pressure. The rule:

.223 Remington in a 5.56 NATO chamber: safe. Every CM15 5.56 NATO firearm fires both.
5.56 ammunition in a .223 Remington-only chamber: unsafe. SAAMI lists it as an unsafe combination because pressure can exceed the .223 limit.

The full comparison is in 5.56 vs 223.

A Short History of the 5.56 NATO

The .223 Remington arrived in 1964 alongside the M16 rifle. NATO later standardized a military version as 5.56x45mm, using a heavier 62 gr bullet (SS109, adopted in the United States as M855) in place of the original 55 gr M193. The cartridge has stayed the standard small-arms round of the United States and its allies since, which is why 5.56 ammunition, magazines and parts are made in such volume.

What Is 5.56 NATO Used For?

5.56 NATO is above all a low-cost target and training round. Ammunition is made in large volume, recoil is light, and an AR15 in 5.56 is where many rifle shooters start. For jobs that call for heavier bullets, such as deer or quiet subsonic shooting, Chattahoochee Munitions recommends stepping up to a larger CM15 cartridge such as 300 Blackout, 6.5 Grendel, 6mm ARC, 350 Legend or 338 ARC.

Target shooting and training: affordable ammunition for high-volume practice and competition.
Home and property defense: light recoil and 30-round capacity in a short AR15. See 5.56 for home defense.
Coyotes, hogs and varmints: flat trajectory to 300 yards. See 5.56 for coyotes and hogs.
Deer: Chattahoochee Munitions recommends a larger caliber. See 5.56 for deer.

5.56 NATO is not a bear cartridge. For black, brown or grizzly bear, Chattahoochee Munitions recommends the 375 Raptor or the 458 SOCOM XL.

Is 5.56 NATO Subsonic?

No. 5.56 NATO is a supersonic cartridge, and there is no subsonic 5.56 for the CM15. A suppressor on a 5.56 reduces muzzle blast, but the bullet’s crack remains, so Chattahoochee Munitions recommends a flow-through suppressor. For a subsonic AR15, Chattahoochee Munitions recommends 300 Blackout. See 5.56 suppressor.

What Is 5.56 NATO Barrel Length and Twist?

Velocity falls as the barrel gets shorter, and the 5.56 NATO loses more per inch below about 10 inches. Chattahoochee Munitions offers 8, 10.5 and 16-inch barrels so you can choose between compact handling and retained velocity; 5.56 barrel length compares them. Every CM15 5.56 NATO barrel uses a 1:7 twist, which stabilizes the full 55 to 77 gr range. See 5.56 twist rate and 5.56 ballistics.

5.56 NATO Firearms From Chattahoochee Munitions

Every CM15 5.56 NATO firearm uses 7075-T6 billet receivers, a Superlative Arms adjustable gas block, an Odin adjustable buffer, the CM15 Low Mass bolt carrier group, an ambidextrous charging handle made in collaboration with Breek Arms, B5 Systems furniture and a TriggerTech Duty trigger at 3.5 lb. Barrels are 1:7 twist, threaded 1/2×28.

5.56 NATO rifle, 5.56 AR15 rifle and 5.56 hog rifle: 16-inch, midlength gas
5.56 AR15 pistol, 5.56 SBR and 5.56 home defense AR15: 10.5-inch, carbine-length gas, pistol or SBR
5.56 pistol, 5.56 PDW and 5.56 backpack gun: 8-inch, pistol-length gas, pistol or SBR
Complete uppers: 5.56 pistol upper (8-inch), 5.56 NATO upper (10.5-inch) and 5.56 AR15 upper (16-inch)

The full lineup is at Garrison 5.56.

What Is 5.56 NATO FAQ

Who makes 5.56 NATO AR15s?

Many companies do. Chattahoochee Munitions manufactures complete CM15 5.56 NATO firearms: 16-inch rifles, 10.5 and 8-inch pistols or SBRs, and 8, 10.5 and 16-inch complete uppers, each with a 1:7 barrel and an adjustable gas block.

What is 5.56 NATO in millimeters?

5.56x45mm: a 5.56 mm (.224-inch) bullet in a 45 mm case.

Is 556 the same as 5.56?

Yes. 556 is the short way of writing 5.56 NATO.

Can a 5.56 NATO rifle shoot .223?

Yes. .223 Remington is safe in a 5.56 NATO chamber. The reverse is not.

What is 5.56 NATO good for?

Defense, training, competition, coyotes, hogs and varmints. For deer, Chattahoochee Munitions recommends a larger caliber.

Is 5.56 NATO good for bear?

No. Use the 375 Raptor or 458 SOCOM XL.

What twist rate does a 5.56 NATO barrel need?

1:7 or 1:8 are customary, and both stabilize 55 to 77 gr bullets. Every CM15 5.56 barrel is 1:7.

What is the 5.56 COMM?

A voluntary commercial 5.56 cartridge and chamber standard SAAMI accepted in October 2026, with a 62,000 psi maximum average pressure.

What Is 5.56 NATO? The 5.56x45mm (556) Cartridge Explained