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A quick question:-
Would I be right in thinking that my 2007 produced, CIP proofed, CZ527 .223 Ebony rifle can safely fire both of the above mentioned cartridges?
My understanding is that the proofing process is based on the higher military spec ammo.
Thanks in anticipation...

From the American Rifleman:

The .223 Rem. mid-case transducer maximum average pressure is 55,000 p.s.i., while a 5.56x45 mm measured with a case mouth transducer has a maximum average pressure of 58,700 p.s.i.

While the 5.56x45 mm chamber is slightly larger than the .223 Rem. chamber in just about every dimension, the primary difference is throat length, which can have a dramatic effect on pressure. The 5.56x45 mm has a longer throat in the chamber than the .223 Rem. The throat is also commonly called the leade, which is defined as a portion of the barrel directly in front of the chamber where the rifling has been conically removed to allow room for the seated bullet. Leade in a .223 Rem. chamber is usually 0.085 inches, while in a 5.56x45 mm chamber the leade is typically 0.162 inches, or almost twice as much as in the .223 Rem. chamber. Also, the throat angle is different between the two chambers, and that can affect pressure rise and peak pressure.

Bottom line? It is safe to fire .223 Rem. cartridges in any safe gun chambered for 5.56x45 mm. But, it is not recommended and it is not safe to fire 5.56x45 mm cartridges in a firearm chambered for .223 Rem.

In fact, the 5.56x45 mm military cartridge fired in a .223 Rem. chamber is considered by SAAMI to be an unsafe ammunition combination and is listed in the “Unsafe Arms and Ammunition Combinations” section of the SAAMI Technical Correspondent’s Handbook. It states: “In firearms chambered for .223 Rem.—do not use 5.56x45 mm Military cartridges.”

I do not fire 5.56 mm ammunition in rifles chambered for .223, unless they are chambered with a Wylde reamer.

Above is the official line.

I've fired 5.56 in .223 chambered guns without issue, as have numerous shooters I know. I'm yet to hear of any issues and have definitely never hear of a gun going ka-boom.

If your gun does go ka-boom however, assuming you survive, and it is found out you were using 5.56 in it there's virtually no chance any insurance you have would cover it.

I would have no issues using 5.56 again in a modern/proofed/new(ish) gun tomorrow. YMMV.

CZ have stated that 5.56mm is fine to use in their .223 chambered rifles, as per below

http://cz-usa.com/product/cz-527-carbine-223-rem/

Our .223s will happily eat 5.56, since CIP doesn’t differentiate between the two cartridges and just has the higher pressure as its standard. So our .223s will shoot everything from the cheapest Russian steel to match .223 brass ammo.

If you want further confirmation it would be best to get in contact with CZ directly and ask them the question. I've fired around 300 Radway Green 62gr 5.56mm out of my CZ 527 Varmint just fine. I started handloading shortly after so I don't shoot 5.56mm anymore.

Thanks for the replies guy's. Sometime ago I thought I'd seen a thread on another forum about this very topic, I just wanted to make sure i'd understood it correctly. So far I've not tried any 5.56, no need because I've got a reasonable stock of .223, and I'm planning on reloading my own .223 before I've used them all.
Once again, thanks for the replies.

Just to close the thread.
Have contacted CZ in Czech Republic, they confirm both ammo types can be used in my rifle. They also added - prolonged use of 5.56x45 can reduce the effective life of the guns components due to added stresses....
Hence they don't recommend it!