Field Note · July 20, 2026
.223 vs. 5.56: The Difference, and Whether You Can Mix Them
Every AR owner runs into this eventually: your barrel is stamped .223 REM but the ammo can in your hand says 5.56 NATO. They look identical and they'll both chamber in your gun. That doesn't mean they're interchangeable. Get this wrong and you're pushing a higher-pressure round through a chamber that wasn't built to handle it.
Same case, different pressure
.223 Remington and 5.56 NATO are external twins. Same case dimensions, same bullet diameter, same overall length. Set two loaded rounds side by side and you probably can't tell them apart without reading the headstamp.
The difference is pressure. 5.56 NATO is loaded to run at higher pressure than .223 Remington, and 5.56 chambers are cut with a longer throat — the bit of freebore before the bullet meets the rifling — to handle it safely. A .223 Remington chamber has a shorter throat that isn't built for that pressure, which is the real reason the two aren't freely interchangeable.
The rule that actually matters
This is the load-bearing safety point, so read it twice.
A rifle chambered in 5.56 NATO can safely fire both 5.56 NATO and .223 Remington ammunition. The chamber is cut for the higher-pressure round, so the lower-pressure .223 round is no problem for it.
A rifle chambered in .223 Remington should fire ONLY .223 Remington. Do not run 5.56 NATO ammo through it. The higher pressure of a 5.56 round in a chamber that isn't built for it can be unsafe.
- 5.56 NATO chamber: safe with 5.56 NATO and .223 Remington
- .223 Remington chamber: safe with .223 Remington only — not 5.56 NATO
- Don't know which you have: load .223 Remington
.223 Wylde: the chamber built for both
There's a third option that removes the guesswork entirely: the .223 Wylde chamber. It's a hybrid design made specifically to handle both .223 Remington and 5.56 NATO safely. If your barrel is marked .223 Wylde, you're covered running either one.
Wylde chambers show up a lot on higher-end AR-15 uppers built for accuracy and mixed-ammo flexibility, so it's worth checking your marking if you're not sure what you've got.
When in doubt, check the barrel
The marking on your barrel tells you what chamber you actually have — .223 REM, 5.56 NATO, or .223 WYLDE. Your owner's manual will back it up. Don't guess, and don't go by what a previous owner or a forum post told you.
If you can't find a clear marking or you're still not sure, load .223 Remington. It's the safe choice in all three chambers, and there's no upside to guessing wrong on this one.