223 vs 5.56: The Urban Legend That Refuses to Die
Let’s Get One Thing Straight
You are not going to blow your rifle up by shooting 5.56 NATO in a .223 Rem chamber.
Take a breath. I know that just shattered a foundational belief for about 60% of box-store counter jockeys and forum trolls across the country.
You’ve probably heard it before. Maybe even said it yourself:
“You can shoot .223 in a 5.56, but not 5.56 in a .223… it’s hotter, it’s overpressure, it’ll kaboom your bolt face clean off!”
Yeah… no. That’s not how any of this works.
Let’s break it down like grown adults with access to actual reloading manuals and not just memes.
So What’s the Real Difference?
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Technically speaking, they’re the same cartridge:
- Same bullet.
- Same external case dimensions.
- Both will chamber and fire in each other’s rifles.
The only meaningful difference? Chamber geometry.
- 5.56 NATO chambers tend to have a longer leade (the freebore in front of the case mouth).
- .223 Rem chambers usually have a tighter throat, which can give better accuracy.
Why the difference? NATO had to account for sloppy brass and dusty M16s. SAAMI didn’t.
“But the Pressure Is Way Higher in 5.56!”
Also no. That’s a myth born of misunderstanding.
Yes, 5.56 NATO has a higher pressure rating on paper. But that’s because it’s tested using a different system than SAAMI.
- You’re comparing apples to oranges.
- Or copper crushers to piezo transducers, if you want to get nerdy.
And remember: 5.56 brass is often thicker, which reduces internal case volume. That actually lowers pressure slightly, not raises it.
Where Did the Myth Come From?
It probably started with:
- Lawyers and liability warnings from rifle manufacturers.
- Internet “experts” repeating something they half-heard in 1996.
- Misunderstood reamer specs and pressure testing jargon.
Let’s be real … if 5.56 posed a serious threat in a .223 chamber, the brass would come with warning tattoos.
This is America. The land of lawsuits over hot coffee and safety warning stickers on chainsaws.
Reloading Manual Breakdown
7th Edition Hornady Handbook (Page 150–157 .223 Remington):
- Originated as a military cartridge in 1957.
- “Readily available surplus brass contributed greatly to the .223’s popularity.”
- No mention of pressure warnings or danger with 5.56.
7th Edition Hornady Handbook (Page 157 – “223 Remington Service Rifle Data”):
- Refers to 5.56 NATO as “.223 Remington Service Rifle.”
- Same case dimensions.
- Treats them interchangeably.
49th Edition Lyman Reloading Handbook (Page 137):
- Title: “.223 Remington / 5.56 NATO” …as a single entry.
- Warns not to shoot .223 in a .222 Magnum — not about .223 vs. 5.56.
- Again: No warning about 5.56 in a .223 chamber.
The Actual Advice
- Want accuracy? Get a .223 Wylde chamber. It shoots both, usually better.
- Running a basic AR? Run what you brung.
- Something won’t chamber? It’s not pressure, it’s probably dirt, a burr the size of the gun counter guy’s ego, or ammo that wouldn’t pass QA at a garage sale.
Final Word
Quit clutching your pearls over .223 vs 5.56.
They’re two names for the same cartridge. One just wears camo and the other wears khakis.
Next time someone tries to tell you they’re “completely different,” hand them a reloading manual and tell them to sit down and read.
Or send them to Carnimore.com.
We’ll explain it slow. With crayons if needed.
Want more myth-busting and real-world rifle info?
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