One of the first things people ask when it comes to the AR-15 platform is: “What exactly is the difference between .223 Remington and 5.56 NATO (5.56mm x 45mm), and why should I care?”. Here, I’ll give you the essential facts related to the differences between these cartridges and the corresponding chamber dimensions. This understanding is very important if you are selecting a barrel, a completed AR-15, buying ammunition, or reloading ammunition for the AR-15.

The Cartridges

The .223 Remington and 5.56 NATO are for all intents and purposes the same in terms of exterior dimensions. The main difference in these cartridges is the thickness of the brass. 5.56 NATO brass is thicker than that of .223 Remington brass. This does two things: first, it reduces the powder capacity slightly (and increases peak pressures) and second, it increases the strength of the case. This means that reloading dies will work the same on 5.56 ammunition as they will on .223 Remington ammunition. 5.56 factory ammunition also features a crimped primer pocket which requires swaging/reaming when reloading so that the primer pocket can accept a new primer.

Here are the specifications for the .223 Remington / 5.56 NATO cartridges:

 

The Chambers

While the cartridges are basically the same except the thickness of the brass, the chambers are slightly different. First off, the 5.56 NATO has a longer leade (distance bullet must travel before engaging the rifling). The extended leade distance is about .025″ – which is a meaningful distance for accuracy based on bullet seating depth. Second, some of the other chamber dimensions are “enlarged” very slightly in order to allow for more reliable feeding/chambering characteristics.

Here is a summary of the differences (positive number means greater in size for 5.56 NATO):

Base diameter	.002″
Shoulder diameter	.0007″
Neck	.001″
Freebore diameter	.0025″
Base to case mouth	.003″
Base to shoulder	.004″
Neck length	-0.002
Freebore length	.025″
Throat angle	-0.6 degrees

In addition the case capacity is different between .223 Remington and 5.56x45mm (NATO) cases: (varies by lot and brass manufacturer)

• 5.56x45mm (NATO): ~30 grains H2O
• .223 Remington: ~30.4 grains H2O

Interoperability of Ammunition

Generally, it is safe to fire .223 Remington in a 5.56 NATO chambered rifle, but it is not necessarily safe to fire 5.56 NATO ammunition in a .223 Remington rifle. This is because 5.56 NATO ammunition is rated for higher pressure (stronger chamber) than .223 Remington. So, if you have a 5.56 NATO rifle, you can shoot both 5.56 NATO and .223 Remington ammunition safely. If you have a .223 Remington chambered rifle it’s best to stick with .223 Remington ammunition. If you are after utmost accuracy, .223 Remington can in some cases be a better way to go due to the shorter leade (assuming factory ammunition is used). If you are handloading for an AR-15, the difference can be negligible.

Summary:

.223 Remington:

• Pro: Can be more accurate chambering (assuming factory ammunition)
• Pro: Brass is readily available new
• Con: Brass is not as strong, can not be loaded as hot
• Con: Can not fire 5.56 NATO ammunition safely

5.56 Nato:

• Pro: Can handle either .223 Remington or 5.56 NATO ammunition safely
• Pro: Brass is stronger, can handle higher pressures
• Con: Can be less accurate (if shooting factory .223 ammunition)

So based on the ballistics you’re after, the interoperability capabilities, and the accuracy goals you have, these criteria should help you to choose which chambering is for you. We’ll get into more nitty gritty details when we move on to handloading in the AR-MPR project.