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5.56 Nato vs. 223 Remington


Many firearms enthusiasts make the mistake of treating the .223 Remington and the 5.56x45mm NATO cartridges as completely interchangeable. The two cartridges look virtually identical to one another, and it would be almost impossible to differentiate between them without examining the headstamps.

Despite this visual similarity, the .223 and the 5.56 are actually drastically different cartridges. Chambers in these two calibers are also built to different specifications. As a result, interchanging the .223 Remington and the 5.56 Nato can have disastrous consequences.


There is almost no visual difference between the .223 Remington vs 5.56x45mm NATO cartridges. What differentiates the two calibers instead is the pressure they create when fired.

5.56 Nato cartridges are loaded with powder to a significantly higher pressure than .223 Remington cartridges. Accordingly, 5.56 Nato chambers are built to withstand higher amounts of pressure. Nato cartridges create more pressure when fired, so Nato chambers are built to withstand more pressure.

You can safely fire a .233 Remington cartridge in a 5.56x45mm NATO chamber. However, you should not fire a 5.56x45mm NATO cartridge from a .233 Remington chamber, because the Remington chamber is not built for the pressure created by the NATO cartridge.


To understand why these cartridges are so different, it’s important to look at the construction of their respective chambers.

The 5.56 NATO cartridge was originally designed for military applications. Because of this, the chamber for the 5.56 NATO cartridge is slightly larger than the .223 Remington. This allows the firearm to operate smoothly and accurately, even if fouling and dirt have built up inside of the chamber. The NATO cartridge can be fired in rougher conditions without compromising reliability or accuracy.

However, the most important difference between the two chambers is the leade.

The leade is the measurement from the bullet’s resting place to the point where rifling is engaged. As a rule of thumb, the shorter the leade, the faster the chamber pressure will rise. If a cartridge designed for a long leade is fired out of a chamber with a much shorter leade, the pressure can increase dangerously fast.

A .223 Remington chamber has about half the leade of a 5.56mm NATO chamber.

This means that firing a 5.56mm NATO cartridge from a .223 Remington-chambered rifle can have disastrous effects.

Typically, the firearm will not explode (though it is possible). It is more common to blow the primer out of the primer pocket from the case. This tiny primer can end up caught in the mechanics of the rifle. Correcting this issue is time-consuming, tedious, and potentially dangerous.

It is not guaranteed that a .223 Remington firearm will malfunction if used with a 5.56 NATO cartridge. However, if the firearm has a little too much fouling or the weather is very hot, it is possible.

Determining What You Have

To avoid this mistake, it’s important to identify your rifle’s chamber and the ammunition you are firing.

If you are using an AR-15, examine the top of the barrel near the muzzle of the firearm. If it says “.223 Remington,” then you can know with certainty what chamber the rifle has. On non-AR-15 rifles, look for this information on the receiver or on the chamber-side of the barrel.

If your rifle says “5.56mm”, that does not mean you can fire all variants of the 5.56 NATO cartridge. The only way to be sure what your rifle is compatible with is to measure the throat yourself, or have a gunsmith measure it. If you are unable to do this, it is safest to avoid firing 5.56x45mm NATO rounds from the rifle.


If your ammunition is labeled on the package as M193 or M855, you definitely have 5.56mm ammunition. If the packaging does not specify, or if you don’t have the package, you will need to examine the bullet itself. If the headstamp on the base of the bullet says “5.56x45mm,” that is what you have. Alternatively, 5.56x45mm cartridges may also be indicated by 2-3 letters and numbers, and a cross inside of a circle.

The headstamp may instead indicate that it is 223 Remington ammunition. If it is unclear, treat the ammunition as 5.56mm.


A .223 Remington cartridge can be fired safely from a .223 Remington chamber or a 5.56 Nato chamber. However, 5.56mm NATO ammunition may only be shot from a rifle with a 5.56 chamber. It is very important to keep this in mind so that you can avoid dangerous and expensive malfunctions.