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Comparing .223 Remington and 5.56x45MM NATO

Posted by Alice Jones Webb on Sep 30th 2019

Comparing .223 Remington and 5.56x45MM NATO

Comparing .223 Remington and 5.56x45mm NATO

The differences between .223 Remington and 5.56x45 NATO have been hammered out and rehashed a million times over. This is a subject regularly bandied about between shooting buddies and complete strangers at gun ranges and on internet forums. Despite the constant chatter, plenty of shooters are still uncertain about the differences between these two popular cartridges.

Many assume they are basically the same and completely interchangeable. Those people are dangerously misinformed.

A Quick History Lesson

The 5.56x45mm cartridge was first introduced in the years following the Korean War. These experimental “small caliber, high-velocity” rounds were developed to be lightweight yet still deliver enough power to be lethal at 500 yards on the modern battlefield.

The round was standardized by NATO in 1963 and officially brought into service by the United States military.

Around this same time, Remington introduced a .223 caliber “civilian” version. This cartridge was standardized by the Sporting Arms and Ammunition Manufacturers’ Institute (SSAMI) in the spring of 1962.

Although, these two cartridges were basically developed concurrently, they are not the same. The 5.56 NATO was engineered for use in war, while the .223 Remington is designed for civilian use. Today, these two cartridges are the most popular centerfire rounds on the planet.

The Similarities

There are some similarities between 5.56 NATO and .223 Remington cartridges. However, those similarities are mostly skin deep. The external dimensions of the cases are nearly identical, and it’s difficult to tell them apart upon first glance.

Here are a few of the most notable qualities shared by .223 and 5.56 cartridges:

● Overall cartridge length

● External case dimensions (although the shoulder profile and neck length differ slightly).

● Case capacity.

● Both shoot 40-90 grain projectiles.

● Both use projectiles that measure .223 inches in diameter with an overall length of 2.26 inches.

The Differences

Appearances can be deceiving. If you look closely, however, you’ll notice a few key differences. For example, although the overall case dimensions are the same, the shoulder profile and neck length are different. 5.56 cartridge cases tend to be thicker in this area to accommodate higher chamber pressures.

Pressure and Velocity

What happens after you pull the trigger on these two cartridges is where the real difference becomes apparent.

Pressure is the primary difference. Compared to the 5.56 NATO, .223 Remington cartridges are loaded to lower pressures and produce slower velocities.

The exact differences are difficult to quantify because pressure is measured differently for each cartridge.

.223 Remington cartridges are standardized by SAAMI (Sporting Arms and Ammunition Manufacturers' Institute). SAAMI measures pressure in pounds per square inch (psi) and is measured at mid-case.

The pressure produced by a 5.56 cartridge is measured at the mouth of the case. These loads are also held to military specs rather than SAAMI specifications.

The different measuring methods prevent an apples-to-apples comparison. The slight difference in the point of measurement can account for difference in pressure measurement of 20,000 psi or more.

However, it is generally accepted that the .223 Remington cartridge is held to a lower pressure regulation than the 5.56 NATO. This is important information for shooters. To put it in basic terms: firing a 5.56x45mm NATO round through a weapon designed for .223 Remington could have perilous consequences.

Chamber Differences

This danger isn’t only due to cartridge differences, however.

The chamber dimensions for the 5.56 are slightly larger than a .223 chamber. The primary difference is the leade dimensions, which has a powerful effect on pressure. The leade (also referred to as throat length) is the space between the front of the chamber and the spot where the bullet engages rifling in the barrel.

Shooting 5.56 NATO from a .223 Remington Chamber

5.56 NATO chambers have leade measurements almost twice that of the .223 Remington. When a cartridge designed for a longer leade is fired out of a chamber with a shorter leade, the result is a rapid and dangerous pressure spike.

This swift jump in pressure can blow the primer out of the primer pocket, causing it to get stuck in the internal workings of your rifle. Shooting 5.56 ammo out of a rifle designed for .223 also increases barrel fouling and general wear and tear on your weapon.

In the worst case scenario, it can even cause serious and catastrophic damage to your weapon (Think exploding firearm). Although a shattered rifle isn’t likely, the possibility is there, and when dealing with harnessing tiny, violent explosions that hurl metal objects nearly three times the speed of sound, it is probably wise to err on the side of caution.

Shooting .223 Remington from a 5.56 NATO Chamber

While experts warn against shooting 5.56 cartridges from a weapon chambered in .223 Remington, doing the opposite doesn’t pose any safety concerns.

However, just because you can do something, doesn’t necessarily mean you should. While .223 Remington ammunition functions in weapons chambered in 5.56, it isn’t the optimal ammunition for those firearms. The lower velocities produced by .223 rounds result in greater bullet drop over distance, which can have a huge effect on accuracy, especially at longer ranges. If you enjoy match competition or popping cans off fence posts at 300 yards to impress your buddies, you should probably stick to shooting 5.56 NATO ammunition.

Summing It Up

Despite their nearly identical appearance, there are several important differences between mil spec 5.56x45mm NATO cartridges and .223 Remington sporting loads. If you have an AR-15 style rifle with a 5.56 chamber, you can safely shoot either .223 or 5.56 cartridges, although you may see a difference in accuracy.

However, if you have a .223 rifle, using 5.56 ammunition can cause excessive wear on your weapon. Although the chance is small, there is still a chance that shooting 5.56 ammo from your .223 Rem firearm could have catastrophic consequences.

If you are unsure about your weapon’s ammo compatibility, be sure to check the owner’s manual or contact the manufacturer for more information.

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