You can safely fire .223 Remington ammo in a rifle marked 5.56. You should not fire 5.56 ammo in a rifle marked only .223 Remington. SAAMI, the group that sets safety standards for American ammunition makers, says the second one is not safe.
The two cartridges look almost the same, and that is why people mix them up. This post explains what is actually different, how to read the markings on your barrel, and what to do if you are not sure. It is general information, not a substitute for your rifle maker’s manual.
5.56 vs .223: the short version
- .223 Remington in a rifle marked 5.56: SAAMI says it is safe to shoot .223 Remington ammunition in a 5.56 military specification chamber.
- 5.56 in a rifle marked .223 Remington: SAAMI says it is not safe to shoot 5.56, 5.56 NATO or 5.56x45mm ammunition in a barrel marked as chambered in .223 Remington.
- Not sure what your rifle is? Read the barrel marking, then the owner’s manual, then call the maker.
All of that comes from SAAMI’s FAQ. SAAMI adds that many non-military 5.56 chambers exist, so a “5.56” stamp alone does not tell you everything. It recommends that you check the owner’s manual or contact the firearm manufacturer to confirm that the chamber meets the military specification.
What is different about the two
The outside of the cartridge looks nearly identical. A technical bulletin posted by Vista Outdoor’s law enforcement division says the cases have basically the same length and outside dimensions (Vista Outdoor technical bulletin, “The Difference Between 223 Rem and 5.56 Military Cartridges”). The differences are in how they are loaded and in the chamber.
Pressure
The military cartridge is loaded hotter. The same bulletin says mil spec 5.56 ammo typically has higher velocity and chamber pressure than .223 Remington. The 5.56 case may also have a thicker sidewall and head to handle that pressure, which leaves less room for powder.
Shooting Times reports that SAAMI sets the .223 Remington maximum average pressure at 55,000 psi, and the 5.56 NATO figure is 62,000 psi (Shooting Times, 5.56 and .223 interchangeability). Those numbers come from two different standards, so use them as a rough guide, not as a head-to-head contest.
The chamber and the throat
The chambers are close but not the same. The difference is in the throat, which the trade calls the leade. This is the section of the barrel just in front of the chamber where the rifling has been cut away to leave room for the bullet.
The Vista bulletin gives typical sizes: about .085 inch in a .223 Remington chamber and about .162 inch in a 5.56 chamber, almost twice as long. Shooting Times says the .223 chamber has a shorter throat and a sharper angle where the rifling starts, while the 5.56 chamber has a longer throat and a shallower angle. Those are typical figures, not a rule for every rifle, so do not use them to measure your own barrel.
Why 5.56 in a .223 chamber is the problem
A hotter round going into a chamber with a shorter throat means more pressure. The Vista bulletin says it is generally known that shortening the leade can dramatically increase chamber pressure, and in some cases the higher pressure could result in primer pocket gas leaks, blown case heads and trouble with the gun’s function.
NSSF’s Let’s Go Shooting explains that firing 5.56 NATO in a .223 chamber can exceed SAAMI pressure limits, and can overwork the action, damage buffer tubes, speed up the bolt and cause dropped primers that jam the action (NSSF, Understanding 223 vs. 5.56).
SAAMI lists the combination as unsafe. The Vista bulletin quotes the SAAMI line: in firearms chambered for .223 Rem, do not use 5.56 military cartridges. Fit is not the test. NSSF says that fitting in the chamber does not make ammunition safe to fire.
Why .223 in a 5.56 chamber is fine, but not perfect
SAAMI calls this combination safe. The Vista bulletin says you can fire .223 Remington in 5.56 chambers, but you will generally have a slight loss in accuracy and velocity compared to the chamber the round was designed for. Shooting Times says accuracy will likely suffer because the lighter bullet is not well supported by the longer throat.
So it is safe, but if you want the best groups, matching the round to the chamber is better.
Bullet weights over time
Bullet weight is another place the names drift apart. Shooting Times says the original .223 Remington load used a 55 grain soft point bullet as a varmint round, and the original 5.56 load used a 55 grain full metal jacket. Military 5.56 loads later moved to 62 grain and then 77 grain bullets, which is part of why the longer throat of the 5.56 chamber exists. If “grain” is a new word to you, our post on what grain means in ammo explains it.
How to check what your rifle is chambered for
- Read the barrel. SAAMI says the cartridge name on the ammunition’s headstamp should match the cartridge name marked on the firearm’s barrel. Look at the barrel, not the receiver.
- Read the manual. If the chamber is made for more than one cartridge, the owner’s manual says which are safe (SAAMI, Recommendations for Safe Ammunition Storage and Handling).
- Call the maker. SAAMI recommends contacting the manufacturer when you are not sure. The chamber type is the maker’s call, not ours.
- Check the ammo box. Read the cartridge name on the box and match it to the barrel marking before you load.
Some rifles are marked “5.56 NATO,” some “.223 Remington,” and some “223/5.56” or similar. NSSF says a “223/5.56” mark indicates a rifle built with a 5.56 NATO chamber that can fire both. Marks vary by maker, so read them as written.
What about the .223 Wylde?
Shooting Times describes the .223 Wylde as a hybrid of the two chambers, and says rifles with it seem to work well with either type of ammunition. It is not a SAAMI standard. If your rifle has one, the maker’s manual is the place to confirm what ammo it was designed to run.
A simple rule for buyers
When in doubt, buy the ammunition that matches what is stamped on the barrel. If the barrel says .223 Remington, buy .223 Remington. If it says 5.56 NATO, either is safe under SAAMI’s guidance, with a small accuracy trade for .223. If the barrel says nothing helpful, do not fire anything until you have an answer from the manual or the maker.
Also keep your ammo in good shape, because heat and damp harm either type. See our post on how to store ammo. For another look at how cartridge types differ, see rimfire vs centerfire.
If you are shopping for ammo for a rifle you already own, bring the rifle’s details or a photo of the barrel marking to the counter at Lakeside Munitions in Cumming. You can also write to us through the contact page before you come in.
