223 -vs- 5.56: FACTS and MYTHS

If you’ve been on the internet long, undoubtedly you’ve read a lot of mis-information about 223 Remington and 5.56 NATO. If you’re like me, you also find it difficult to find answers and specifics related to these two cartridges and rifle chamberings. The 5.56 NATO chamber is “larger” than the 223 Remington chamber- but my how much? What is the “real deal” related to 223 Remington and 5.56 NATO ammunition interoperability?

In this post, I’ll share some information I’ve gathered from multiple industry partners, and by means of good old fashioned research (like weighing cases in the Ultimate Reloader lab). If you are in the market for 223 or 5.56 brass, please visit my industry partner Capital Cartridge!

How 223 Remington and 5.56 NATO Came to Be

While this is by no means exhaustive, I thought I’d summarize the events that lead us to this point in time- this age where there’s a plethora of different rifles, chamber specifications, load data, information, and mis-information all related to the AR-15 and the 223 Remington cartridge.

Here’s a rough timeline of events:

Early 1950s

Eugene Stoner creates a radical new rifle design around the 308 Winchester cartridge called the “AR-10” (there were other designs as well).

Late 1950s

The US Army, through an organization called CONARC, publishes the requirements for the Army’s next-generation battle rifle.

1963

Remington arms releases a new cartridge called “223 Remington” along with a version of the Remington 760 chambered for this new cartridge.

1964

The US Army adopts the AR-15 and designates it the M16 with semi-auto and full-auto firing modes. The US Army also creates a military-specific variant of 223 Remington called “5.56 NATO” (5.56x45mm)

1994

The US Military officially adopts the M4, a newer rifle variant based on the M16/AR-15.

223 -vs- 5.56 Ammunition

There are some differences you’ll notice right off the bat between .223 Remington and 5.56 NATO Ammunition:

  • On the case rim, you’ll notice “223” stamped on 223 Remington brass, where 5.56 NATO brass may only have a date, initials (Ex: FC) and one or more symbols
  • On the case rim, you’ll notice that 5.56 military ammunition will have a crimp around the primer pocket  (not the case for new 5.56 cases hand loaded in the civilian market)

Here’s a side by side comparison of the case rims for 223 Remington and 5.56 NATO: (click/tap to enlarge)

There may be other differences externally as well. 5.56 ammunition will have a cannelured and crimped bullet, where 223 Remington ammunition may or may not have a cannelure or crimp. Dimensionally, these cartridges are essentially identical on the exterior with the exception of some bullet profiles used for 5.56 NATO ammunition.

Here are some similarities between .223 Remington and 5.56 NATO Ammunition:

  • Same external case dimensions (shoulder transition may have different radius)
  • Same cartridge overall length
  • Same case trim-to length
  • Both cartridges were designed/optimized for 55 grain bullets
  • Both use small rifle size primers

223 Remington Cartridge Diagram:

5.56 Nato Cartridge Diagram:

Both diagrams above created by Francis Flinch.

You can see above, the only difference is the radiused shoulder-to-neck transition for the 223 Remington cartridge (sharp for 5.56 NATO). This difference does not affect function in any of the common 223-variant chamberings including 5.56 NATO.

The case capacity is also slightly different:

  • 5.56 NATO case capacity: 28.5 grains H2O
  • 223 Remington case capacity: 28.8 grains H2O (+1.1% compared to 5.56 NATO)

As with most factors related to 5.56 and 223, actual case capacity will vary. In summary: the exterior dimensions of 223 Remington and 5.56 NATO ammunition are essentially the same. 223 Remington and 5.56 NATO cases are essentially the same, perhaps with 1% difference in case capacity between the two.

Confusion: Pressure Ratings and Testing

Where much of the confusion comes when trying to understand the differences between 223 Remington and 5.56 NATO ammunition is when reading the published maximum allowed pressure for each cartridge.

You may see data published that looks like this:

223 Maximum pressure (SAAMI)                    380 MPa (55,000 psi)

5.56 Maximum pressure (EPVAT)                    430.00 MPa (62,366 psi)

…and this data would make it appear that 5.56 NATO ammunition is certified “to a higher pressure” than 223 Remington ammunition. That is actually not the case! It’s only when we observe the US Military’s updated pressure testing and rating data when we see a parallel:

5.56 Maximum pressure (SCATP 5.56)           380.00 MPa (55,114 psi)

You’ll note here that different testing methods are specified:

  • SAMMI (Sporting Arms and Ammunition Manufacturers’ Institute) pressure testing: requires drilling a hole in case a specified distance back from the case mouth. Pressure is measured externally by means of gas which escapes via hole in case. This method is complicated, but high in accuracy.
  • EPVAT (Electronic Pressure Velocity and Action Time) pressure testing: measures pressure from rifle’s chamber/barrel area ahead of location of case mouth. This method is quicker and easier compared with the SAMMI pressure testing method, but is not as accurate.
  • SCATP pressure testing: A new military standard for cartridge pressure testing that is essentially equivalent to the SAMMI method.

Here’s a comparison of the SAMMI/SCATP pressure test location compared to the EPVAT pressure testing location within the rifle’s barrel/chamber:

And here’s a picture of the pressure testing apparatus used for these kinds of tests (image courtesy PCB Piezotronics):

For SAMMI and SCATP tetsing, the location of the pressure port (and hole drilled in case) is specified and measured relative to a “conformal line” as shown in this diagram:

The advantages of the SAMMI/SCATP pressure testing methods are cleaner data and more accurate/reliable results. And it’s when you compare the SCATP-5.56 pressure rating to the SAMMI 223 Remington pressure rating that things become clear:

  • 5.56 Maximum pressure (SCATP 5.56) 55,114 psi
  • 223 Maximum pressure (SAAMI) 55,000 psi

The conclusion: 223 Remington and 5.56 NATO have essentially the same maximum working pressure rating.

So what about the “interoperability issues” between 223 Remington and 5.56 NATO then? These are real, some of the time. Let’s first compare the chambers between .223 Remington and 5.56 NATO.

223 Remington and 5.56 NATO Chamber Differences

There is some information that’s easy to find on the internet, and some that must be gathered from reputable online sources and industry players (large companies, smaller reamer grinding shops, and everything in between). I’ll note here that chamber dimensions seem to be somewhat “open to interpretation” by reamer manufacturers and firearms OEMs. So I’m not going to state anything as universal fact here: these are numbers that I feel best represent the differences between 223 Remington spec chambers and 5.56 NATO military spec chambers. In short, reamers and chambers will vary, and the most important thing is how firearms are certified and tested.

As seen in the video, here are the differences in chamber dimensions between 223 Remington and 5.56 NATO (click/tap to enlarge)

Yes, the 5.56 NATO chamber is “a couple thousandths larger here and there” – but the most important difference is the freebore (lead) length. The freebore is the smooth portion of the bore (with no rifling, oversize compared to the bullet) that accounts for variations in bullet ogive/length, and gives the bullet some “room” to exit the cartridge slightly before the throat guides the bullet into the rifling. With twice the freebore length (0.050″ for 5.56 NATO compared to 0.025″ for 223 Remington) the 5.56 NATO chamber can safely accomodate cartridges that produce slightly higher peak pressure, and bullets that have extended ogives. Other than this freebore difference, the rest of the “deltas” can account for enhanced reliability for 5.56, and can cause 5.56 chambered rifle accuracy to suffer somewhat compared to otherwise equivalent 223 Remington chambered rifles.

The differences between the 223 and 5.56 chambers are evident in the documentation that FORSTER provides for their headspace gauges. Here are the differences between the 223 Remington and 5.56 NATO headspace gauge guidelines:

You can read the full document from FORSTER HERE.

You’ll notice that the 223 Field (the max safe chamber for 223 Remington, beyond typical chamber dimensions) and the 5.56 NATO “MAX” dimensions are different, with the 5.56 NATO MAX being 0.004″ longer than the 223 Remington FIELD dimension. This is consistent with the diagram above which shows the base to shoulder dimension with the same difference between the two chamberings.

223/5.56 Hybrid Chambers

Also note that there are several “hybrid chamber” designs out there that “split the difference” between the 5.56 NATO chamber specification and the 223 Remington chamber specification. These include the 223 Wylde, and Noveske’s hybrid chamber. It may or may not be safe to shoot 5.56 NATO ammunition in 223-variant chambers: check  your manufacturer’s literature to be sure. What these hybrid chambers claim to do is offer the interoperability of the 5.56 NATO chamber with accuracy closer to that of the 223 Remington chamber. I haven’t tested these hybrid chambers to test the accuracy claims- but if you have, please leave a comment to tell us about your experiences!

223 Remington and 5.56 NATO Ammunition Interoperability

SAMMI makes it quite clear- you can shoot 223 Remington ammunition in a 5.55 chamber, but you shouldn’t shoot 5.56 ammunition in a 223 Remington chamber:

So why is this? We already talked about how the cases and exterior ammunition dimensions are nearly the same. It has to do mostly with bullet profile!

The military has used a wide variety of bullet types in the M16 and M4. The most common use-case is the 55 grain FMJ 5.56 NATO cartridge, but on the other end of the spectrum you have the 64 grain tracer 5.56 NATO cartridge. This tracer ammunition utilizes a bullet that has a different profile as seen here: (click/tap to enlarge)

Because the 64 grain tracer bullet is less dense, the overall length of the bullet has to be increased, but lengthening the bullet requires a faster twist rate in order to remain stable in flight. So the ogive of the bullet (the curved portion that first meets the rifling) is also pushed forward in order to squeeze more volume out the bullet. This extended ogive is part of the reason it isn’t considered safe to shoot 5.56 NATO ammunition in a 223 Remington chamber- it’s because the military specifies 5.56 NATO ammunition relative to the 5.56 NATO chamber NOT the 223 Remington chamber. So you can’t assume any 5.56 NATO ammunition is safe to shoot in a gun chambered for 223 Remington.

Reloading 223 Remington and 5.56 NATO Ammunition

Reloading 223 Remington and 5.56 NATO ammunition is essentially the same as loading most other bottleneck rifle cartridges. You will use the same press, powder measure, shellplate or shellholder, and so on and so forth.

Reloading dies are the same for 223 Remington and 5.56 NATO because as we’ve already discussed, the exterior dimensions are essentially the same. Here’s a LEE RGB 223 die set that’s also certified to work with 5.56 NATO cases:

When reloading 223 Remington and 5.56 NATO cartridges, there are a few things to keep in mind:

  1. If you are loading previously fired military 5.56 cases, you’ll need to swage or ream the primer pockets to remove the crimp.
  2. Load data for 223 Remington and 5.56 NATO is not the same! Use the appropriate load data for the case you are loading.
  3. Check your twist rate! For 223 Remington and 5.56 NATO, twist rates will vary from 1:14 (slow) to 1:7 (fast). Longer/heavier projectiles will require a faster twist rate. Always check the bullet manufacturer’s data to confirm.
  4. 22 caliber case mouths are small- I would suggest cleaning the lube from your sized cases before charging with powder to avoid powder sticking/bridging at the case mouth.

Conclusion

In short- rifles chambered in 5.56 NATO are most versatile and perhaps most reliable. Rifles chambered in 223 Remington can offer enhanced accuracy, but are potentially less reliable in a semi-automatic rifle like the AR-15. Hybrid chambers attempt to offer the best of both worlds. It is safe to shoot 223 Remington cartridges in any 223-variant chamber, but it is not always safe to shoot 5.56 NATO ammunition in a 223 Remington chamber. The perceived differences in pressure rating between 223 Remington and 5.56 NATO are mostly the effect of different pressure testing methodologies, and in reality 223 Remington and 5.56 NATO cases are basically equivalent.

As with all shooting and reloading: be safe, and always check to make sure your ammunition and rifle “agree”. Most of all, have fun!

If you are looking for high-quality 223 or 5.56 brass at an affordable price, check out these offerings from Capital Cartridge!

Thanks,
Gavin

About the author

4 thoughts on “223 -vs- 5.56: FACTS and MYTHS”

  1. Well done and thorough! I shoot 223 Wylde and had been warned years ago to not shoot 5.56 ammo in it. Your article explains why very well. Thanks again!

    1. The 556 brass is heavier (thicker) and therefore has less internal volume. Loading it the same as your 223 brass will result in higher pressures. Stick with 223 brass for 223 loads.

      1. I have not found that to be true- I’ve measured the brass wall thickness with a ball micrometer, and weighed the cases side by side. In some cases 223 cases weighed more, some weighed less compared with 5.56 NATO.

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