Initially I never actually kept track of what I shot out of which AR because it was so inexpensive I never considered reloading for it. Then I got a Winchester chambered in .223 to use the brass from the AR's in. I never paid attention to whether it was 5.56 or .223 I was shooting out of it, and never noticed any signs of over pressure or anything else, that is until I bought the Remington XP 100 in .223. The first shot told me something was wrong when I had to open the bolt with a block of wood. Obvious sign of over pressure. But me being me I thought it was perhaps just the brand of ammo and pressed on with a different brand with the same result. So I tried another brand and had no sign of over pressure. I now know what I did and I will say that Remington builds a pretty tough gun to withstand what amounts to serious over pressure loads and still shoot just fine.The point being that if it says 5.56 on your barrel your good to use either. If it says .223 on your barrel....stick with .223 or carry a block of wood to open your bolt.