223 vs 5.56: What You Can Safely Fire

Here is the whole answer, and it is not symmetrical.

• .223 Remington in a 5.56 NATO chamber: safe. This is the direction that works. You may see slightly reduced accuracy because the bullet has a longer jump to the rifling, and that is the entire cost.
• 5.56 NATO in a barrel marked .223 Remington: SAAMI says this is not safe. That is not a hedge or a lawyer sentence we are repeating. It is the position of the standards body that publishes the .223 Remington chamber drawing, and it is stored in our compatibility graph as a directed edge from the .223 chamber to 5.56 ammunition, sourced to the SAAMI FAQ.

If you remember one thing, remember that the permission runs from the more generous chamber to the less generous cartridge, and never back. A 5.56 chamber accepts both. A .223 chamber accepts one.

This is also why our database has no "compatible" boolean anywhere in it. Compatibility here is a directed edge with a verdict, a mechanism and a citation attached, because a symmetric flag would be wrong in both of the two most commonly confused cartridge pairs in the United States. The .308 and 7.62 NATO pair reverses this rule entirely, and that section is at the bottom of this page.

The two cartridges have identical external case dimensions and an identical 0.224 inch bullet diameter. They will both chamber in both rifles. Nothing about handling them tells you which is which. The difference is inside the barrel, in a few thousandths of an inch of geometry called the leade or throat: the unrifled section ahead of the case mouth that the bullet crosses before it engages the rifling.

The .223 Remington leade is roughly 0.085 inches. The 5.56 NATO leade is roughly 0.0975 inches. The 5.56 chamber gives the bullet more room to accelerate before it has to swage into the rifling.

Fire a 5.56 round, which is loaded to a higher pressure to begin with, in the shorter .223 throat, and the bullet meets the rifling sooner and while more of the powder is still burning. Chamber pressure climbs above what the load was designed to produce. That is the mechanism, and it is why the direction matters: the same round in the longer throat has the room the load assumed it would have.

The pressure ceilings we carry make the size of the gap visible: .223 Remington at 55,000 psi and 5.56 NATO at 62,366 psi. Treat that comparison as directional rather than exact, because the two figures come from different standards bodies measuring at different points with different equipment, and the raw numbers are not strictly interchangeable. The throat geometry is the mechanism. The pressure numbers are the evidence that the margin is not large.

Two honest caveats. Many modern .223-marked barrels are cut with generous throats and shooters fire 5.56 through them routinely without incident. And the failure, when it happens, is usually a blown primer or a hard extraction rather than a destroyed rifle. But "usually fine" is not what a chamber marking means, and the SAAMI position on the pairing is unambiguous. We report the standard, not the anecdote.

The cartridge you own is printed on the ammunition. The chamber you own is stamped on the barrel, usually on the side near the chamber end or on the top of the barrel extension. The receiver marking is not the chamber marking. Rifles are sold with markings on the lower receiver that describe the model rather than the barrel, and barrels get swapped. If your rifle came with a manufacturer specification sheet, that is the second source. The barrel stamp is the first.

You will see four common markings:

• 5.56 NATO, sometimes written 5.56x45. Accepts both 5.56 and .223.
• .223 Rem. SAAMI says 5.56 does not belong in it. Feed it .223.
• .223 Wylde. A hybrid chamber, designed specifically to take 5.56 pressures safely while keeping .223 accuracy. This is not a marketing name, it is a real chamber reamer specification, and it accepts both.
• 5.56 NATO Mil-Spec or a NATO cross-in-circle proof mark, on military-pattern barrels. Same as 5.56.

If a barrel is unmarked, treat it as unknown rather than as 5.56. An unmarked barrel is the one case where the cheap answer, which is to shoot only .223 through it until you can confirm the chamber, is also the correct one.

Everything above is a pressure argument with real but usually survivable consequences. This next one is different in kind, and it is the reason the compatibility graph exists at all.

.300 Blackout uses a 0.308 inch bullet. A 5.56 or .223 bore is 0.224 inches. The .300 Blackout case is a 5.56 case necked up, so it feeds from standard 5.56 magazines, fits standard lowers, and will chamber in a 5.56 upper. Every mechanical cue that normally stops a mistake is absent.

Firing a 0.308 inch projectile into a 0.224 inch bore does not raise pressure by a percentage. It obstructs the bore. The rifle can come apart, and people have been seriously injured this way. It happens most often to shooters who own both, share magazines between them, and grabbed the wrong one at the bench.

In our database this pairing carries the verdict catastrophic in both directions, from a .223 chamber and from a 5.56 chamber, and it is flagged to be blocked from search results outright rather than merely ranked lower or shown with a warning. It is the one class of result we refuse to return.

The practical defense is physical, not cognitive. Mark .300 Blackout magazines distinctly, store the ammunition separately, and never have both cartridges out on the same bench at the same time. Do not rely on reading the headstamp under range lighting.

People generalize from .223 and 5.56 to the other famous NATO pairing, and the generalization is backwards.

• 7.62x51 NATO ammunition in a .308 Winchester rifle: generally safe. The NATO round is loaded to a lower pressure than commercial .308.
• Commercial .308 Winchester in a military 7.62x51 chamber: our graph rates this unsafe. Military chambers are cut with generous headspace, and commercial .308 brass has thinner case walls than military 7.62 brass, so the combination can rupture cases.

So the safe direction on the .223 and 5.56 pair runs from the commercial cartridge into the military chamber, and the safe direction on the .308 and 7.62 pair runs from the military cartridge into the commercial chamber. Exactly opposite. Anyone who has learned one pair as a rule of thumb and applied it to the other has learned the wrong thing.

That is why we store directed edges with a verdict, a mechanism and an authority on each one rather than a single interchangeability flag. There is no rule of thumb here that survives contact with the second example.

None of this is a safety guarantee. It is drawn from standards-body and manufacturer specification and it is informational. Chamber markings, barrel configuration, your firearm manufacturer''s instructions and local law govern what you may safely fire.