I'm sure this has been asked and answered a million times.
what's the diff ?

and I think they will interchange one way but not the other... correct?

.223 Remington is rated for a max pressure of 50,000 CUP, while 5.56 Nato is rated at 60,000.

Also NATO chambers have a long leade. SAAMI chambers are tighter and have a short leade this means they will yield dangerously high pressures in guns using the military spec ammunition.

So you can shot both in a 5.56 chambered weapon, but not 5.56 in a .223.

that's what I needed to know thanks!

From my experience I have found 223 to be more accurate.

The difference cannot be limited to the different pressures (SAAMI vs MIl). You need to consider terminal performance you need for your application. NATO and the UN were not thinking about you, when they created the ammunition regulations. Don't assume NATO is good for your needs.

The .223 offers more options, with better terminal performance in some cases. Don't limit yourself to mil spec ammo. It was designed by the government. You also need to undestand the limits of your gun. Can it shoot heavier rounds than 55 or 62 grain?

Randy

Most NATO ammo is not made to be match ammo, but to be used in machine guns and assault rifles, where you would be firing many rounds instead of one shot. There is Match 5.56 ammo, but it is marked as such and generally is not available to the public.

Also understand that there is different grades of regular 5.56 ball ammo, with the cheapest and least accurate being used for practice ranges and the better being used for combat and rifle qualification.

The main reason i say that is most "target" AR's are only 223.

I found this from Wikipedia to answer your question

While the 5.56mm NATO and .223 Remington cartridges and chamberings are very similar, they are not identical.

Military cases are generally made from thicker brass than commercial cases; this reduces the powder capacity (an important consideration for handloaders[6]), and the NATO specification allows a higher chamber pressure. NATO EPVAT test barrels made for 5.56mm NATO measure chamber pressure at the case mouth, as opposed to the location used by the United States civil standards organization SAAMI. The piezoelectric sensors or transducers NATO and SAAMI use to conduct the actual pressure measurements also differ. This difference in measurement method accounts for upwards of 137.9 megapascals (20,000 psi) difference in pressure measurements. This means the NATO EPVAT maximum service pressure of 430 megapascals (62,000 psi) for 5.56mm NATO, is reduced by SAAMI to 379.21 megapascals (55,000 psi) for .223 Remington.[7] In contrast to SAAMI, the other main civil standards organization C.I.P. defines the maximum service and proof test pressures of the .223 Remington cartridge equal to the 5.56mm NATO.

The 5.56mm NATO chambering, known as a NATO or mil-spec chamber, has a longer leade, which is the distance between the mouth of the cartridge and the point at which the rifling engages the bullet. The .223 Remington chambering, known as SAAMI chamber, is allowed to have a shorter leade, and is only required to be proof tested to the lower SAAMI chamber pressure. To address these issues, various proprietary chambers exist, such as the Wylde chamber (Rock River Arms)[8] or the Armalite chamber, which are designed to handle both 5.56mm NATO and .223 Remington equally well. The dimensions and leade of the .223 Remington minimum C.I.P. chamber also differ from the 5.56mm NATO chamber specification.

Using commercial .223 Remington cartridges in a 5.56mm NATO chambered rifle should work reliably, but generally will not be as accurate as when fired from a .223 Remington chambered gun due to the longer leade.[9] Using 5.56mm NATO mil-spec cartridges (such as the M855) in a .223 Remington chambered rifle can lead to excessive wear and stress on the rifle and even be unsafe, and the SAAMI recommends against the practice.[10][11] Some commercial rifles marked as ".223 Remington" are in fact suited for 5.56mm NATO, such as many commercial AR-15 variants and the Ruger Mini-14, but the manufacturer should always be consulted to verify that this is acceptable before attempting it, and signs of excessive pressure (such as flattening or gas staining of the primers) should be looked for in the initial testing with 5.56mm NATO ammunition

Nice info DavKind