Differences Between 5.56 Nato and .223 Remington
5.56x45 NATO vs .223 Remington
The question you and thousands of others have asked: “What is the difference between the .223 Remington and the 5.56x45 NATO round”? Well, the good news is: not much, but the small changes can make a huge difference. As far as the actual case goes the only major difference is the angle at which the case body and shoulder meet up. The .223 Remington has a sharper transition, while the 5.56x45 NATO has a slight curve to it. The other difference is the case wall thickness; the .223 Remington has a neck thickness of 1.1mm (0.0433 in) and the 5.56x45 NATO 1.14mm (0.0448 in).
| .223 Remington | 5.56 NATO |
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Chamber Differences
The real differences between the two are in the chambers and how they are designed. The 223 chamber is basically a tighter spec (primarily in the throat area) and the ammunition that is loaded and tested to 223 Remington spec. is loaded for a max. working pressure of 55,000ps. The 5.56 Nato chambers (there are actually two different standard spec. Nato chambers) have a bigger in diameter and a longer throat. So in essence the Nato chambers are looser. Also when ammo is made to Nato spec. it is loaded and tested in Nato pressure test barrels and loaded to a max working pressure of 58,000 psi. So you shouldn’t take Nato spec. ammo and run it thru a 223 Rem chambered barrel/gun as it will drive up pressures of the Nato spec ammo even more. We’ll come back to the larger freebore/throat in just a moment.
In the very early days of the M16 rifle program the ammo that was issued was a 55gr round known as M193. The M193 was the designation the US military gave to the 55gr .223 Remington dating back to the initial acceptance in 1963. In the 1970s NATO was looking to standardize a round to use between all countries, and through the XM777 program, FN Herstal out of Belgium developed the SS109 cartridge. Through trial testing, it was put up against the British 4.85x49 and the German 4.73x33 cartridges. SS109 was determined to be the best and on October 28, 1980 under STANAG 4172, becoming the official standard round of NATO. It was adopted by all NATO countries, and some non NATO countries, and in the US was given the designation of M855. With the adoption of the new NATO cartridge modifications were made to the camber as part of the standardization. The length and the diameter of the freebore were changed as well as the angle of the throat. The shallower throat angle is what allows for higher pressures because it's a more gradual transition. You have to remember that in a military setting reliability and getting rounds down range is far more important then shooting the tightest groups possible.
The larger freebore allows significantly more reliability especially under full auto fire due to the in-built allowance for ammunition inconsistencies and different projectile designs, allowing all 5.56 in use by NATO countries to function in 5.56x45mm rifles. Imagine the logistical nightmare it would be if a US soldier were fighting next to a British soldier, and one of them has an M4, and the other an L85, respectively. The US soldier runs out of ammo, but the British guy next to him can't give him his ammo because it was loaded differently despite it being the designated universal NATO round. That just wouldn't work for multinational (or joint) operations.
One thing we did not touch on, but is an important thing to consider if you are wanting to build a rifle to chase accuracy, the 5.56 NATO is going to be less accurate than the .223 Remington not only because of the longer freebore, but more importantly the larger diameter of the freebore. Having a freebore with a larger diameter means the projectile has time to yaw before being engaged with the rifling. Any runout the loaded ammunition has and or shooting a poor quality bullet it basically already has a premature wobble to the bullet and because of the longer and looser throat dimensions this can make bullet alignment worse when the bullet makes the jump into the rifling of the bore and have a negative effect on accuracy. The actual time is microseconds, but that can be the difference between .7 MOA and 1.2 MOA. If you are wanting to build a rifle to chase the tightest groups possible, then getting a rifle with a 5.56x45 NATO chamber might not be the best thing for you. There is something you can use to get the best of both worlds, though.
| 5.56 NATO | .223 Remington | |
|---|---|---|
| Parent case | .223 Remington | .222 Remington |
| Case type | Rimless tapered, bottleneck | Rimless tapered, bottleneck |
| Bullet diameter | 5.70 mm (0.224 in) | 5.70 mm (0.224 in) |
| Land diameter | 5.56 mm (0.219 in) | |
| Neck diameter | 6.43 mm (0.253 in) | 6.43 mm (0.253 in) |
| Shoulder diameter | 9.00 mm (0.354 in) | 9.00 mm (0.354 in) |
| Base diameter | 9.58 mm (0.377 in) | |
| Rim diameter | 9.60 mm (0.378 in) | 9.60 mm (0.378 in) |
| Rim thickness | 1.14 mm (0.045 in) | 1.1mm (0.0433 in) |
| Case length | 44.70 mm (1.760 in) | 44.70 mm (1.760 in) |
| Overall length | 57.40 mm (2.260 in) | 57.40 mm (2.260 in) |
| Case capacity | 1.85 cm3 (28.5 gr H2O) | 28.8 grain H2O (1.87 ml) |
| Freebore length | 1.499 mm (.059 in) | 0.635 mm (.025 in) |
| Freebore diameter | 5.7531 mm (.2265 in) | 5.6896 mm (.224 in) |
| Throat Angle | 1° 13 minutes 20 seconds (1.222°) | 3° 10 minutes 36 seconds (3.177°) |
| Rifling twist | 1:7 | 1:9-1:12 |
| Primer type | Small Rifle | Small Rifle |
| Maximum pressure (SAAMI) | 55,000 psi (380 MPa) | |
| Maximum pressure (CIP) | 62,366 psi (430.00 MPa) | |
| Maximum CUP | 52000 CUP | |
| Maximum pressure (EPVAT) | 62,366 psi (430.00 MPa) | |
| Maximum pressure (SCATP 5.56) | 55,114 psi (380.00 MPa) |
.223 Wylde
I promise I'm not just trying to confuse you, the .223 Wylde chamber was designed to be a happy medium between the .223 Remington and the 5.56 NATO in an attempt to increase accuracy. One thing we touched on already was the 5.56x45 NATO chamber not only has a longer freebore, but also a larger diameter. The downside to that is has a negative effect on accuracy. The larger diameter of the freebore allows the projectiles to yaw before engaging in the rifling. That is not a problem in war where your rifle only needs to be “minute of man” for standard troops. However, it does become a problem if you want to chase accuracy. A gentleman by the name of Bill Wylde came up with the concept of the .223 Wylde chamber in the mid 1980s, which essentially is a 5.56x45 NATO chamber, but with the freebore of .223 Remington. Where the confusion comes in, is that .223 Wylde chamber reamers can be ordered with a number of different freebore lengths. The reason for this is if you are wanting to shoot a heavier (longer) projectile, for example a 82 grain bullet, you would need that extra space before the rifling. Firing those heavier projectiles in a very short freebore of say the .223 Remington could have high pressure issues as the bullet needs to be seated deeper into the cartridge case.. So make sure you check with the rifle or barrel manufacturer to see what freebore size they are using in their .223 Wylde chambers.
Conclusion
Simply put, the 5.56x45 NATO has a larger freebore to accommodate for the heavier (longer) projectiles, and to make the weapon more reliable in full auto fire. The change was made with the introduction of the 62gr SS109 when it was adopted by NATO. It allows for larger variation of projectiles and manufacturing inconsistencies from around the world. The larger freebore does have a negative effect on accuracy, but on a military rifle it's a non-issue. You are not shooting for accuracy, you are shooting at a man sized target. Most rifles today, especially if you are buying an AR-15 style rifle, is going to be chambered in 5.56x45. You should always check with the manufacturer to see what your rifle is chambered in, though. Sometimes you can look on the barrel and it will be marked to designate the chambering. When ordering a custom barrel ALWAYS make sure you have a copy of the reamer specs it was made with so you know the exact specifications of the barrel. Since there is typically a minimal standard that manufacturers have to follow they can deviate from one to another.
Is the .223 Wylde enough of a change to swap your barrel over to? Honestly, probably not, but that is going to a decision you will have to make. If your main goal is to chase MOA and you are handloading then it would make sense to go with the .223 Wylde chamber.
Chamber Specifications
| Print 514: Standard .223 Remington | Print 516: 5.56x45 NATO (SS109) |
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| Print 517: M193 NATO (1:12 twist) | Print 561: Standard .223 Wylde |
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Special Thanks
I wanted to give a special thanks to the following for helping me with this article. Without them this would not have happened.
