What is the difference btwn 5.56 and 5.56x45mm (is there a difference)? I see ammunition marked both on shelves side by side.

No, they are the same. The difference comes when it's labeled 223 Rem or 5.56. 223 Rem is loaded to 55,000 psi, and the 5.56 is loaded to 62,000 psi. 5.56 ammo should not be shot in a 223 Rem chamber.

^

I have lots of once fired 5.56 brass if anyone is interested in trading for brass in other calibers.

http://en.wikipedia.org/wiki/5.56%C3%9745mm_NATO

Once again---5.56 ammo is NOT loaded hotter than .223 Remington. If you load ammo for your AR15 that's too jot for your bolt gun then you are loading too hot for your AR. The NATO round is said to generate 62000 psi but this is using the EPVAT standard. When NATO ammo is measured per SAAMI methods, it turns in 55000 psi just like .223 Remington.

The pressure issues associated with 5.56 ammo are due to the differences between the .223 chamber and the 5.56 chamber.

Then why is there 223 Rem data and 5.56 data, and the 5,56 data have 2 to 4 grains more powder than the 223. More powder equals more pressure. The 5.56 is rated as a hotter round. Always has been.

I load 26.0 gr varget with 60 grain hornady v-max bullets. Is this a 223 or 5.56 round?

Well. If you can accept the premise that a 5.56 NATO cartridge is a very specual .223 round loaded to a certain specification (IMO that's what it is), then you are loading .223 Remington rounds. Since they're yours I suppose you can call them whatever you like. So what DO you call them??

It's just one of those myths that grow legs and refuse to die.

Ok. Let me type exactly what is written in my trusty Hornady reloading manual 9th edition, page 157, since I have it right here with me.

"The main difference between the 5.56 NATO and the 223 Rem is the operating pressure and chamber throat. SAAMI max pressure for the 223 Rem is 55K psi. The 5.56 NATO is loaded to 60K psi. Chamber throat for the 223 Rem is shorter than the 5.56 chambers as well."

That's what Hornady says. Now if you look at reloading data (almost all sources), you will find that there is data for both the 223 Rem and 5.56. The 5.56 will have about 2 to 4 grains more powder comparing it with the same bullet and the bullet will be seated out a little further in the 5.56, (about .020 to .1", depending on the bullet).

There's all kinds of methods to pressure test a round. If you find a method that tests both the same, the 5.56 round should be the hotter round. I don't know how else to explain that a 5.56 round is a hotter round than a 223 Rem.

Myth? Take a Rem 700 bolt rifle chambered in 223 Rem and go fire about 20 rounds of 5.56 LC ammo, and tell me how well it works for you. You will get a quick lesson on pressure!

I'm out.

Like I said, there's a lot of misinformation out there. It likely all originated with someone failing to make clear that the standard specified by NATO will show a higher value. Next thing you know, everyone believes the 5.56 is SUPPOSED to be loaded hotter. It is NOT.

And THAT is due to the chamber dimension differences.

5.56×45mm NATO

5.56×45mm NATO with measurement, left to right: bullet, empty case, complete round with bullet in case
Type Rifle
Place of origin  United States
Service history
In service Since 1962
Used by NATO
Wars Since Vietnam War
Production history
Designer Remington Arms
Specifications
Parent case .223 Remington
Case type Rimless, bottleneck
Bullet diameter 5.70 mm (0.224 in)
Neck diameter 6.43 mm (0.253 in)
Shoulder diameter 9.00 mm (0.354 in)
Base diameter 9.58 mm (0.377 in)
Rim diameter 9.60 mm (0.378 in)
Rim thickness 1.14 mm (0.045 in)
Case length 44.70 mm (1.760 in)
Overall length 57.40 mm (2.260 in)
Case capacity 1.85 cm3 (28.5 gr H2O)
Rifling twist 178 mm or 229 mm (1 in 7 in or 9 in, originally 1 in 14 in)
Primer type Small rifle
Maximum pressure (EPVAT) 430.00 MPa (62,366 psi)
Maximum pressure (SCATP 5.56) 380.00 MPa (55,114 psi)

I'm not sure how well this image can be seen, it's a screenshot saved as a .jpg file.

The above is a page out of a 1994 publication of the Department of the Army. Chamber pressure for M193 ammo is given as 52,000 psi. I'm not sure why it's listed so low, but maybe it was published before a more modern standard was adopted. I'll be looking for the answer to that.

A little tidbit to add to the confusion. European standard 223 rifles must meet CIP and not SAAMI standards so they are tested a 62000 PSI by the Piezoelectric method. That does not mean the leade is as long as for 5.56. Most European manufactures have adopted the longer leade from the 5.56 for safety reasons. The only way to know for sure is a chamber cast or a Gage tool. You can change the leade in a 223 to 5.56 with a specialty reamer if you find the leade is too short.

So...what this is showing you is that you can load a .223 cartridge and take a pressure measurement per SAAMI standards and get a pressure figure of 55000 psi, or you can employ the EPVAT method as prescribed by NATO and the pressure measurement will be 62000 psi. There is NO difference in the pressure allowable for 5.56 NATO round and the .223 Remington round.

I would strongly suggest that if you handload for an AR chambered in 5.56 NATO, do NOT operate on the premise that this ammo is hotter than SAAMI spec .223 Remington. You will be playing with fire.

Now, if you ask me to explain the nonsense you see in some reloading manuals...all I can say is that as I learn more about this hobby I see more and more and more myth and falsehood being passed off as fact. Sometimes it's not real obvious. The trouble is that certain things become established as "fact" and it's all but impossible to eradicate those ideas.

The last thing I'll say about it is that I am not posting this stuff to discredit anyone at all...all I care about is determining what really IS fact. There are matters of opinion that can be debated til the sun doesn't shine, and that's fine and dandy (and should be fun as long as folks mind their manners). But there are also matters of FACT that are important enough to settle for everyone's benefit. Ego does not matter. Fact does. I've fallen victim to misinformation just as much and often as anyone else has, and as uncomfortable as that can be to admit you have to decide for yourself whether or not you want to be CORRECT. I've had to change my tune many a time because that's what it takes to be correct at times. No one is born knowing all this stuff.

I wonder how many thousands and thousands of rounds of the cheap 5.56 has been fired in .223's. My son's friends aggravate me all the time when I take them predator hunting by always shooting 5.56 FMJ (.223's) and I have to look for the critter that runs 50 yards.
I have tried to find where there has been an issue documented with a 5.56 fired in a .223 that caused a problem but I cannot. Lawyer's have a powerful influence. JMO

Saw this and thought it was pretty good explanation without the usual the sky is falling.

http://www.luckygunner.com/labs/5-56-vs-223/