Can I Shoot 5.56 NATO in a .223 Remington Chamber?

The Throat Is the Problem

The .223 Remington and 5.56 NATO cartridges are dimensionally similar enough that 5.56 rounds will chamber and fire in a .223 Rem rifle. The issue is not whether the round fits — it's what happens when it fires.

The critical difference is the leade (also called the throat): the short, unrifled section of the barrel between the case mouth and the start of the rifling. In a .223 Remington chamber, the leade is approximately 0.085 inches long. In a 5.56 NATO chamber, it's approximately 0.162 inches — nearly twice as long.

That longer throat gives the 5.56 NATO bullet room to accelerate before it engages the rifling. When you fire a 5.56 round in a .223 Rem chamber, the bullet contacts the rifling much sooner, which restricts forward motion, builds pressure faster, and can spike chamber pressure above what the .223 Rem chamber is designed to handle.

Test Methods Differ — But the Risk Is Real

SAAMI specifies a maximum average pressure of 55,000 psi for .223 Remington using a copper crusher method. NATO specifies a maximum of approximately 58,000 psi for 5.56 NATO using the EPVAT piezo test method.

These test methods measure pressure differently, which makes direct comparison imprecise. The NATO figure, converted to a SAAMI-equivalent measurement, is often cited around 58,000–62,000 psi — meaningfully above the .223 Rem SAAMI maximum.

In a .223 Rem chamber, the earlier bullet-to-rifling contact adds a pressure spike on top of the already higher base pressure. The total effect can push pressure well past the .223 Rem design limit — not guaranteed to cause a failure, but not a margin you want to erode repeatedly.

The .223 Wylde Chamber

The .223 Wylde is a hybrid chamber specification developed by Bill Wylde. It combines the tighter bore dimensions of the .223 Remington (for accuracy) with a longer throat closer to the 5.56 NATO spec (for pressure safety). It safely handles both .223 Rem and 5.56 NATO ammunition.

Many AR-15s marketed as ".223 Remington" rifles are actually manufactured with a .223 Wylde chamber. The barrel marking may say ".223 Rem" while the chamber is cut to Wylde spec. This is common among quality AR-15 manufacturers. Check your manufacturer's documentation or contact them directly.

Bolt-action rifles chambered in .223 Remington are more likely to have true SAAMI .223 Rem chambers than AR-15-pattern rifles. The risk of overpressure is highest in bolt guns with tight commercial .223 Rem chambers, particularly with hot 5.56 military surplus ammunition.

Can I Shoot It?