If you have a gun that is not clearly marked as to what is the specified cartridge that it will safely fire, have it checked by a gunsmith or a competent firearms professional, as even the cartridge identification markings on the gun itself may not be correct on some firearms, as I will elaborate on later.
I bring up this topic because the proliferation of the AR-15 'style' of tactical weapons has greatly popularized the 5.56x45mm NATO cartridge that many of these guns are chambered for. But if you have a tactical rifle or carbine that appears to be chambered for the 5.56 cartridge, you had better make SURE. Some of these look-alike guns are actually chambered for the .223 Remington cartridge, and may or may NOT be clearly marked as such.
There were a number of AR-15s and AR-15 look-alikes sold by several manufacturers to the public that were incorrectly marked as being chambered for the 5.56x45 NATO round, but were actually chambered for the .223 Remington. So... again, if you have purchased a new or used rifle or carbine that is marked as being chambered for the 5.56 round, it may be advisable to check the serial number and call the factory, or take it to a gunsmith who can accurately measure the chamber and throat of the barrel BEFORE you fire the weapon, or even load it, for that matter.
Side by side, these two cartridges look identical to the naked eye. Even the diameter of the bullet of these two cartridges is the same. I checked this out by looking into my old Speer reloading manual. The actual diameter of the bullet used to reload both the 5.56x45mm NATO and the .223 Remington measures in at 0.224 inches. When I checked a gun forum online, they verified the Speer reloading manual is correct on the 0.224” diameter bullet for both those cartridges (and also among several other popular small bore cartridges).
But the assembled 'cartridges' are not exactly the same in several respects.
Let's look at some of the data that outlines some of the differences between these two popular cartridges:
Let's start with the brass cases used to make up the two cartridges. The brass used to produce the 5.56x45 NATO cartridge is thicker (and likely stronger) than the brass used to produce the .223 Remington.
Most all of the 5.56x45 NATO cartridges produced have a slightly longer overall length (by a few thousandths of an inch) than the commercial .223 Remington cartridges. And there are also critical differences in the chamber dimensions of these two high-power rifles (and carbines) that these loads are designed for.
There is a free space, referred to as the leade or throat, where the chamber meets the barrel of a rifle just short of where the groves and lands of the rifling begin. This space is most critical. If the assembled cartridge is too short, the leade will be too great, with a negative effect on velocity and accuracy. But if the leade space in the gun is too short, and if the bullet is forced into the rifling as it is pushed into the chamber, when the gun is fired, excessive (DANGEROUS) pressure could likely be built up which may not only seriously damage the firearm, but could seriously injure, or in the extreme case, even kill the person firing the weapon!
When we are talking about high pressure here, we are dealing with from approximately 40,000 pounds per sq. inch when the .223 Remington cartridge is fired, as compared to more than 50,000 – 55,000 pounds per sq. inch when the 5.56x45 NATO cartridge is fired. That's why the margin for error is so critical when we are dealing with cartridge and chamber measurements.
With a .223 Remington-chambered carbine or rifle, the leade measures 0.085” as compared to the relatively longer 0.162” leade in a gun correctly chambered for the 5.56x45 mm NATO cartridge.
So... in a nut shell:
Never Put A 5.56x45mm In A Gun Chambered For The .223 Remington!
While a .223 Remington round can 'normally' be safely fired in a gun chambered for the 5.56x45mm, you will lose ballistic efficiency. * But there have also been incidents (mostly with reloaded ammo) where excess free space (leade) caused some cartridge cases to rupture. *
And with ALL firearms, if there is Any Doubt at all as to whether you have the EXACT cartridge that can be safely fired in that weapon, Don't Even Load It!
Here's an excellent article on the subject: .223 Remington vs. 5.56 NATO: What You Don’t Know Could Hurt You
And this video should also be of help in understanding the issue:
Stay Safe and Protect Our Second Amendment!
Gary Miller
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