There is a good and thorough article out there where a bunch of "civilian" 5.56 and .223 rem factory ammo was pressure tested to verify or debunk the 5.56 vs. .223 remington debate. Pressures were all over the place with different lots of both 5.56 and .223 from various manufacturers. Some .223 had higher pressures than 5.56 nato and vice versa. There was very little of the ammo that had pressures that were cause for concern.
As far as safety issues go, more than anything it's going to come down to the individual gun rather than the ammo itself. If you have a tight chamber and short throated barrel it's going to be a bigger issue than a rifle with a large sloppy chamber and short throat. As always there are differing opinions. Manufacturers will always be on the "don't do it" side because of liabilities. You just have to decide for yourself if it's worth it or not.
This is going to be slightly off track but not really. I've seen a lot of "kabooms" in my lifetime, most of them were Glocks (an entirely different subject). Personally I have had one of my guns "kaboom", it was a brand new Ruger mini-14, twenty years ago and it was caused by the ammo being used. It wasn't because I was shooting 5.56 nato ammo but rather I was shooting "remanufactured" ammo by a company named MIWALL. You've probably seen them at gun shows, they are a California company and they are a big ammo vendor at gun shows. They take up about half an aisle and sell everything from factory ammo to their own "new" and "reman" ammo. I had bought 1,000 rounds of .223 in an ammo can from them.
Fast forward to father's day and I was out shooting with my father in law. I put a few rounds down range with the Ruger and then handed it to him. He fired off a couple and then "BOOM" a round went off when the bolt was out of battery. Part of the case was left in the chamber, the case head blew into the scope tube, the barrel was blown out at the chamber, stock split in half and a thirty round mag blown apart (during the Clinton ban era). My father in law had a speckled face and glasses, I'm just glad he was wearing glasses. Fortunately most of the pressure blew out down through the magazine well.
It was obvious that this Kaboom was caused by poor reloads, improperly sized brass that didn't allow the bolt to close completely. The ammo company "MIWALL" refused to do anything about it. Ruger replaced the gun even though I knew and they also stated that it was solely caused by the ammunition. ( I don't know of a gun company that has better service than Ruger does ) Here's the cherry on top: digging through that ammo can I discovered the bottom third of it was steel cased Russian Wolf ammo.
I have quite a few guns chambered in both 5.56 and .223; I don't shoot much factory ammo but rather I reload large amounts at a time on a Dillon. I don't build a load to a particular rifle, I feed all my guns from the same batch. Most of them shoot MOA regardless of the chambering. I don't load to max pressures and I trim all brass as well as full length size it on a small base resizing die.
The sun, moon, earth and stars have to all be aligned before 5.56 ammo is going to blow up a .223 remington rifle. I've heard of it happening but never seen it first hand. Long story short I would personally worry more about the company and the quality of ammo rather than if it's stamped 5.56 or .223;
