.223 vs 5.56: A Comparison

The first modern rimless centerfire rifle cartridge produced for commercial use in the U.S. hit the market in 1950: the .222 Remington. Introduced as a varmint and benchrest target round for Remington’s Model 722 bolt-action rifle, it used a bottlenecked case of some 43.2mm in length with a .224 caliber (5.7mm) bullet, which led to its sale overseas as the 5.7×43.

To make a long story short, Remington in 1958, after moving away from the bolt-action market and working with ArmaLite to come up with a cartridge that met a pending U.S. Army requirement for a light carbine, pushed the length of the case out to 47mm to increase the amount of propellant in the powder charge, resulting in rifles chambered in .222 Remington Magnum. 

Then, to split the difference between these two cartridges as the military tests continued, later dropped down to a 45mm length case with a shorter neck and the shoulder moved back slightly to accommodate a .223 caliber bullet and christened this middle child offshoot the new and improved .223 Remington.

Big Green submitted the brand new .223 Remington ammo cartridge to the Sporting Arms and Ammunition Manufacturers’ Institute, or SAAMI, the folks that publish safety, reliability, and interchangeability standards for commercial firearms and ammo, way back in 1962. 

Enter the first .223 Remington rifle

The next year, Remington added their first available .223 Rem chambered rifle, a variant of the pump-action Model 760 Gamemaster– a rifle that had been offered in .222 ammunition since 1958– to their catalog. Colt, meanwhile, had acquired the rights to ArmaLite’s new gee-whiz modern rifle, by that time known as the AR-15, and it was chambered for .223 ammunition as well.

When the Colt AR-15 hit the consumer market in 1963, it was pitched as a hunting rifle chambered in .223 Remington. Select-fire variants were at the same time being sold to the military in a carbine that would become the M16, which was adopted in 1964.

Americans fighting in Vietnam in the 1960s and 1970s, who were using the assorted early variants of select-fire AR-15/M-16 rifles issued to them, loaded their weapons with what was termed by the military as “Cartridge, 5.56mm ball, M193.” This was simply .223 Remington ammunition, although there was an evolving controversy over propellant types used during this era for the military contract.

Notably, the pressure levels published by SAAMI for the commercial .223 Rem ammo were echoed in Army publications of the 1970s such as MIL-C-9963E/F. The use of M193 ball, with its traditional 55-grain FMJ bullet, continues to some extent today, largely for guns bought by Washington for overseas military aid to allies who may still be using older M16A1s.

Fast forward to the development by Belgian-based FN Herstal of the Minimi light machine gun in the early 1970s. The Belgians at the same time put big R&D brains into a new 5.56x45mm cartridge using a 62-grain bullet with a steel core to meet NATO’s new ammo requirement to penetrate steel helmets at 600 meters. 

The resulting cartridge, which required a slightly different chamber design for safety, became known as the SS109, and was standardized by NATO for small arms use in 1979.

The M855 Green Tip evolves into the EPR

When the Minimi was adopted by the U.S. military in 1982 as the M249 Squad Automatic Weapon, chambered in 5.56 NATO ammo (using the SS109 round, which the U.S. Army dubbed the M855 “green tip”), Uncle Sam soon quickly updated the old M16A1 standard to the new M16A2 which included a chamber profile and barrel twist rate for 5.56 NATO, going so far as to swap out the uppers on legacy rifles to make an easy upgrade to the new ammunition. 

The M16A4 and M4/M4A1 standards further updated the line.

Since 2010, the U.S. Army has put the M855 green tip ammo to pasture in favor of the more barrier-blind M855A1 Enhanced Performance Round, or EPR, which can still use the 5.56 NATO chamber (although has shown to be more finicky in some older magazines due to the cartridge’s geometry.)

And here we are today.

The difference between the two rounds comes down to their ability to handle maximum pressure. SAAMI lists the maximum average pressure for .223 Remington ammo in a .223 chamber as 55,000 psi with bullets ranging from 35- to 77-grains traveling anywhere from 4,000 to 2,785 ft. per second velocity.

The rough European version of SAMMI, the Belgian-based C.I.P., recognizes a higher max pressure curve for .223 ammo of around 62,000 psi– which can lead to some confusion– and we’ll get into that.

No 5.56 SAAMI Spec?

What is the SAAMI spec for 5.56 ammo? They don’t have one as that spec was developed by NATO but published pressures outside of SAAMI are listed as high as 62,000 psi for standard loads while proof rounds, developed to test barrels at 125 percent pressure, touch the 78,000 psi threshold.

Curiously, C.I.P. also lists the max pressure for standard 5.56 NATO ammo as being in the 62,000 psi range. Here is where we note that the Europeans there at C.I.P. chamber measure pressure using a different method than SAAMI does, hence the higher numbers.

Chamber dimensions matter

Speaking to chamber types, going past the round, where the difference between .223 and 5.56 is the greatest is in the chamber profile used at the throat of the barrel for the caliber. 

For instance, an AR-15 barrel marked as chambered in .223 will have a shorter throat into the rifling than that of a 5.56 NATO standardized barrel (something that first appeared in 1979) which usually has about .125 inches of additional freebore. 

This difference can cause increased pressure when 5.56mm NATO cartridges are fired in a .223 Remington chamber. For instance, when using “hot” 5.56 NATO rounds in a .223 Rem-marked rifle with a tighter chamber than what the military round was designed for, pressures can push into the 70,000 psi area in worse case scenarios since the mil-spec round doesn’t have the extra length to expand into, which isn’t good, especially in long term use.

Is it dangerous to mix 5.56x45mm NATO and .223 Rem?

Can you shoot 5.56mm NATO rounds from a rifle chambered for .223? Bottom line. Direct from SAAMI: “NO! It is not safe to shoot ‘5.56’ ‘5.56 NATO’ or ‘5.56x45mm’ (‘5.56’) ammunition in a firearm with barrel marked as being chambered in .223 Remington for a number of reasons,” warns the organization, who has maintained this position since 1980.

“This can result in serious injury or death to the user and/or bystanders, as well as damage to the firearm.”

Can I shoot .223 in my 5.56 Rifle or Pistol?

While it is seen as unsafe to fire 5.56 ammo in a firearm that has a barrel marked as being chambered in .223, the reverse is not true.

 “It is safe to use SAAMI-compliant 223 Remington ammunition in firearms with a barrel marked as chambered in ‘5.56,’” says SAAMI. “If you are unclear about which ammunition is appropriate to safely use in your rifle, consult the firearm owner’s manual or contact the firearm manufacturer for further guidance.”

The exterior case dimensions of both cartridges, at least in factory loads, are the same, and barrels/firearms chambered for either .223 or 5.56 will accept the rounds vice-versa. 

This is also true in magazines. As discussed in more detail in other sections, in the interest of safety it is advised by firearms industry experts that “5.56” marked ammo may be unsafe to fire in a rifle or pistol listed as being chambered for .223 Remington. 

However, .223 cartridges can generally be used safely in a “5.56” marked firearm.

Warning signs

When in doubt, consult the manufacturer. A warning sign that you are dealing with too much pressure is if you start seeing “flattened” primers on ejected cases or notice the primer itself pop out of the case when fired. That pressure is trying to go somewhere other than the chamber, an important red flag.

When talking chamberings, firearms that use the SAAMI-spec commercial .223 Remington chamber will typically have a tighter throat and lend towards better accuracy. Just make sure, in an abundance of caution, that you run .223 marked ammo through the build.

Try not to succumb to the temptation of bulk pack 5.56 rounds — which are, generally speaking, going to be cheaper.

Accuracy vs. Adaptability

For a more all-purpose or heavy-use firearm, 5.56 chamberings may be a better option, especially if planning to run lots of ammo through it over the years. These guns will still have good practical accuracy but may not be competition tack drivers. 

When talking chamberings, firearms that use the SAAMI-spec commercial .223 Remington chamber will typically have a tighter throat and lend towards better accuracy. Just make sure, in an abundance of caution, that you run .223 marked ammo through the build.

The .223 Wylde chambering was designed to pull off the magic trick of walking the tightrope between both the 5.56 NATO and .223 Remington chambers’ freebore diameter/throat dimension, and leade angle, and doing it accurately while handling the 5.56mm NATO round higher pressure without issue. 

This provides a chamber with very tight tolerances that allow the use of ammunition types as well as special loads with longer bullets, going into the 80-grain range. 

In short, when you come across an AR chambered in .223 Wylde, you have a universal adapter when it comes to using either .223 Rem or 5.56 NATO ammo, without giving up the accuracy of the first and living in the safe pressure range for the second.

So, to reiterate, while the exterior physical dimensions of both commercial .223 Rem and 5.56 NATO are identical, 5.56 ammo can often be loaded to higher pressures than .223 and, as such, firearms with a 5.56 chambering are larger in key areas to accept that greater potential pressure. 

Generally speaking, while you can fire .223 ammo in a firearm labeled for 5.56, it may not be safe to do the opposite and fire 5.56 ammo in a gun labeled for .223 as the latter could have a tighter chamber.

Safety first when it comes to metal objects going off at up to 60,000 psi just a few inches in front of your face.

Ballistic differences between the two cartridges are moot, but 5.56 can often be had at better prices due to its tendency toward bulk production and packing.