Short answer: look at what your barrel is marked. A barrel marked 5.56 NATO can safely fire .223 Remington, per SAAMI, and a .223 Wylde barrel is made to fire both. A barrel marked only .223 Remington must not fire 5.56: SAAMI says its shorter throat raises pressure and calls that combination unsafe. The two cartridges look alike and use the same .224 inch bullet, but 5.56 is loaded hotter: SAAMI's new 5.56 Commercial standard sets 62,000 psi against 55,000 psi for .223 Remington.
Which ammo in which chamber
| .223 Remington | 5.56 NATO | .223 Wylde | |
|---|---|---|---|
| What it is | SAAMI commercial cartridge and chamber | Military cartridge; no public standard, per SAAMI | A chamber, not a cartridge, designed by Bill Wylde |
| Pressure | 55,000 psi maximum average (SAAMI Z299.4) | 5.56 Commercial: 62,000 psi (SAAMI, June 2026) | Fires both cartridges |
| Fire .223 Remington? | Yes | Yes, per SAAMI, in a military-spec 5.56 chamber | Yes, per its makers |
| Fire 5.56? | No: SAAMI calls it unsafe | Yes | Yes, per its makers |
| Throat | Shorter | Longer | Tighter than 5.56, per Wilson Combat |
Sources: the SAAMI FAQ, SAAMI's 5.56 Commercial standard (issued June 2026), the SAAMI Z299.4 centerfire rifle standard, and Wilson Combat on .223 Wylde, read October 2026.
What caliber is 5.56?
5.56x45mm means a 5.56 mm bullet in a case about 45 mm long. SAAMI's 5.56 Commercial drawing shows a .2245 inch bullet and a 1.760 inch (44.70 mm) case. .223 Remington uses the same .224 inch bullet; the difference is pressure and the chamber, not the bullet.
Why you cannot fire 5.56 in a .223 Remington chamber
SAAMI says a barrel marked .223 Remington has a shorter throat into the rifling than a 5.56 barrel, so 5.56 ammunition raises pressure, and it calls the combination unsafe. Its June 2026 5.56 Commercial standard repeats that only .223 Remington ammunition should be used in a firearm marked only .223 Remington.
The new SAAMI 5.56 Commercial standard
In June 2026 SAAMI published a voluntary "5.56 Commercial" (5.56 COMM) cartridge and chamber standard at 62,000 psi. SAAMI notes there is no public standard for 5.56 NATO, so it cannot make blanket compatibility statements between military 5.56 and this new standard, and makers do not have to mark guns or ammunition 5.56 COMM. If your barrel is marked something other than 5.56 COMM, ask its maker about compatibility.
What is .223 Wylde?
A chamber designed by Bill Wylde for AR-15s that takes both cartridges. Wilson Combat says its throat is tighter than a 5.56 NATO chamber for accuracy, and that it works with all .223 and 5.56 ammunition. It is not a SAAMI cartridge: there is no .223 Wylde entry in SAAMI's centerfire rifle standard, and you still buy .223 or 5.56 ammunition for it.
.223 Wylde and 5.56 barrels, with today's prices
Lowest price across the stores we track:
Faxon Match Gunner 5R .223 Wylde barrel, 16 in
Pros
- .223 Wylde chamber for both cartridges
- 5R rifling, Gunner profile
Cons
- Barrel only; needs a gas block and tube
Wylde
Ballistic Advantage Premium .223 Wylde barrel, 18 in SPR
Pros
- .223 Wylde, 1:8 twist
- SPR contour, 1/2x28 threads
Cons
- 18 in, longer than a carbine barrel
Wylde, longer
FN America 5.56 16 in mid-length barrel, chrome lined
Pros
- 5.56 NATO chamber
- Chrome-lined bore
Cons
- Barrel only
5.56
.223 and 5.56 ammo, with today's prices
Federal American Eagle .223 Remington 55 gr FMJ (20)
Pros
- .223 Remington
- Safe in .223, 5.56 and Wylde chambers
Cons
- FMJ: practice, not hunting
.223
Federal American Eagle 5.56 NATO 55 gr FMJ (20)
Pros
- 5.56 NATO load
Cons
- Not for barrels marked only .223 Remington
5.56
Hornady Frontier 5.56 NATO 55 gr M193 (20)
Pros
- M193-style 5.56 load
Cons
- Not for barrels marked only .223 Remington
5.56 M193
Compare all barrels and ammunition by price, see our 5.56 ballistics chart and barrel profiles and twist rate, and plan the rifle in the AR-15 builder.
Frequently asked questions
Can I shoot 5.56 in a .223 rifle?
Not if the barrel is marked only .223 Remington. SAAMI says the shorter throat raises pressure and calls it unsafe.
Can I shoot .223 in a 5.56 rifle?
Yes, per SAAMI, in a military-specification 5.56 chamber. Because some commercial "5.56" chambers differ, SAAMI says to check your manual or the maker.
What is the difference between 5.56 and .223?
Same .224 inch bullet and similar case, but 5.56 runs at higher pressure: SAAMI sets 55,000 psi for .223 Remington and 62,000 psi for its new 5.56 Commercial standard. 5.56 chambers also have a longer throat.
Is .223 Wylde better than 5.56?
It fires both cartridges; Wilson Combat says its tighter throat helps accuracy. That is a maker's claim, and a 5.56 chamber also fires both.
What caliber is 5.56?
A 5.56 mm (.224 inch) bullet in a 45 mm case: 5.56x45mm.
What is 5.56 Commercial?
A voluntary SAAMI cartridge and chamber standard published in June 2026, at 62,000 psi. SAAMI says it is not the same as any military or NATO specification.
