You do realize that 5.5.56x45mm NATO and .223 Rem are virtually identical as 5.56mm was based on the .223 cartridge, right? The bullets/projectiles they use are exactly the same and the dimensional differences between their cases are so minute that you would need precision calipers to tell the difference. Most rifles chambered for one or the other will feed both just fine. Sometimes you'll get a picky rifle that will only want to shoot what it's chambered for, but most will work with both no problem. It's not like an AR with .223 Remington stamped on the barrel is going to blow up in your face if you fire a 5.56x45mm cartridge through it. Now, trying to fire a 5.45x39mm cartridge in a rifle chambered for .223 or 5.5.56x45mm would be a more dangerous affair since it's not related to .223/5.56mm. The biggest issue you might run into you might run into firing 5.56mm in a .223 or vice-versa is stuck cases during extraction and shift in point of impact on the target.