5.56 vs 223 comes down to chamber and pressure. The two cartridges have the same outside case dimensions and fire the same .224-inch bullets, but 5.56 NATO ammunition is loaded to higher pressure and its chamber has a longer leade. That makes the rule one-way: .223 Remington is safe in a 5.56 NATO chamber, and 5.56 ammunition is unsafe in a .223 Remington-only chamber. Chattahoochee Munitions manufactures its CM15 AR15 firearms and uppers in 5.56 NATO, so each one fires both.
5.56 vs 223 at a Glance
| Specification | 5.56 NATO (5.56x45mm) | .223 Remington |
|---|---|---|
| Bullet diameter | .224 inch | .224 inch |
| Case length | 1.760 inches | 1.760 inches |
| Maximum overall length | 2.260 inches | 2.260 inches |
| Chamber leade | Longer (about .162 inch) | Shorter (about .085 inch) |
| Pressure | Higher; military specification, and 62,000 psi for the commercial 5.56 COMM | 55,000 psi SAAMI maximum average |
| Fires the other safely? | Yes, fires .223 Remington | No, do not fire 5.56 |
| Standard | NATO military; SAAMI 5.56 COMM (2026) | SAAMI |
The .223 Remington standard and SAAMI’s unsafe-combinations list are published by SAAMI. The 5.56 COMM figures come from SAAMI’s October 2026 acceptance, as reported by American Rifleman.
Why Is 5.56 vs 223 a Safety Question?
The leade is the unrifled section between the case mouth and the point where the bullet meets the rifling. In a .223 Remington chamber that jump is shorter, so a 5.56 NATO cartridge seats its bullet closer to the rifling. Combined with the 5.56 load’s higher starting pressure, the shorter leade can drive pressure past the .223 limit. Possible results are flattened or leaking primers, stuck cases and damage to the firearm. SAAMI lists 5.56 ammunition in a .223 Remington chamber as an unsafe combination. That applies to military surplus, commercial 5.56 and the new 5.56 COMM.
Going the other way is fine. A .223 Remington cartridge in a 5.56 NATO chamber simply has a longer jump to the rifling and runs at its own, lower pressure. Accuracy and velocity can be slightly lower than in a .223 chamber, but it is safe.
How to Tell a 5.56 Chamber From a .223 Chamber
- Read the barrel marking. A barrel marked 5.56, 5.56 NATO or 5.56×45 fires both. A barrel marked only .223 or .223 Rem fires .223 only.
- Read the box. Ammunition marked 5.56, 5.56×45, XM193 or XM855 is 5.56. Ammunition marked .223 Remington is .223.
- When the marking is unclear, ask the maker. Every Chattahoochee Munitions CM15 5.56 barrel is a 5.56 NATO chamber.
5.56 vs 223 Ballistics
Ballistically, 5.56 vs 223 is close. Federal publishes 3,165 fps for its 55 gr American Eagle 5.56x45mm (XM193) load and 3,020 fps for its 62 gr .223 Remington full metal jacket. Matched bullet for bullet, 5.56 loads usually run somewhat faster because of the higher pressure, and the gap shrinks or grows with the load and the barrel. The table below is a Chattahoochee Munitions calculation from Federal’s published velocities and ballistic coefficients (100-yard zero, 2.5-inch sight height, standard atmosphere, 10 mph full-value crosswind).
| Range | 5.56x45mm 55 gr FMJ-BT: velocity / energy / drop / wind | .223 Rem 62 gr FMJ-BT: velocity / energy / drop / wind |
|---|---|---|
| Muzzle | 3,165 fps / 1,223 ft-lb | 3,020 fps / 1,255 ft-lb |
| 100 yd | 2,766 fps / 934 ft-lb / 0 / 1.3 in | 2,708 fps / 1,009 ft-lb / 0 / 1.1 in |
| 200 yd | 2,401 fps / 704 ft-lb / -2.1 in / 5.4 in | 2,416 fps / 804 ft-lb / -2.3 in / 4.4 in |
| 300 yd | 2,064 fps / 520 ft-lb / -10.2 in / 13.0 in | 2,143 fps / 632 ft-lb / -10.5 in / 10.6 in |
| 400 yd | 1,757 fps / 377 ft-lb / -26.6 in / 25.0 in | 1,889 fps / 491 ft-lb / -26.4 in / 19.9 in |
Bullet weight and shape matter more than the 5.56 or .223 label. Barrel length matters too: these are test-barrel velocities, and shorter barrels are slower. See 5.56 ballistics and 5.56 barrel length.
5.56 vs 223 vs .223 Wylde
.223 Wylde is a chamber, not a cartridge. It uses a 5.56-style leade with a tighter .223-style freebore diameter, so it fires both safely and is popular on match barrels. Chattahoochee Munitions CM15 barrels use the 5.56 NATO chamber.
5.56 vs 223: Which Should You Buy?
For an AR15, buy a rifle chambered in 5.56 NATO. It fires both cartridges, so you can buy whichever ammunition is available and suits the task: 5.56 for training and defense loads, .223 Remington for many varmint and match loads. Only a .223-only bolt action or a .223-only AR15 limits you to one.
5.56 vs 223 in the CM15
Chattahoochee Munitions manufactures the CM15 in 5.56 NATO as a 16-inch midlength rifle (5.56 NATO rifle), a 10.5-inch pistol or SBR (5.56 SBR) and an 8-inch pistol or SBR (5.56 PDW), each in three Cerakote finishes. Complete 8, 10.5 and 16-inch uppers fit a standard AR15 lower; see the 5.56 NATO upper. Every barrel is a 5.56 NATO chamber with a 1:7 twist, and every firearm has a Superlative Arms adjustable gas block and a TriggerTech Duty trigger. The lineup is at Garrison 5.56.
Comparing 5.56 with other AR15 cartridges? See 22 ARC vs 223, 6mm ARC vs 223 and 6.5 Grendel vs 5.56.
5.56 vs 223 FAQ
Who makes a 5.56 NATO AR15 that fires .223?
Chattahoochee Munitions manufactures complete CM15 5.56 NATO rifles, pistols, SBRs and uppers. Every one has a 5.56 NATO chamber, so it fires both 5.56 and .223 Remington.
Is 5.56 the same as 223?
No. The cases look the same, but 5.56 runs at higher pressure and needs a 5.56 chamber.
Can you shoot 223 in a 5.56?
Yes. .223 Remington is safe in a 5.56 NATO chamber.
Can you shoot 5.56 in a 223?
No. SAAMI lists 5.56 ammunition in a .223 Remington chamber as unsafe.
Is 556 the same as 5.56?
Yes. 556 is shorthand for 5.56 NATO.
Is 5.56 more powerful than 223?
Usually a little faster with the same bullet, because of the higher pressure. Bullet choice and barrel length matter more.
What is the 5.56 COMM?
A voluntary commercial 5.56 standard SAAMI accepted in October 2026, at 62,000 psi. It does not change the .223 rule: no 5.56 ammunition in a .223-only chamber.
Do 5.56 and 223 use the same magazine?
Yes. Both feed from standard AR15 magazines.