I bought my first rifle about 2 weeks ago- a Bushmaster XM15 E2S.

When I purchased the rifle and up until an hour or so ago I though that 5.56 & .223 were the same round and were interchangable in use.
Well I just finished reading a ammo FAQ, and it indicated that ar's have different barrels/chambers for .223 & 5.56 NATO.  I checked my barrel and it says 5.56 NATO on it.

I've put about half and half .223 & 5.56 though my gun(but less than 1k total rounds so far...).

So my question(s) is 1) Have I or will I do any harm to my carbine in using .223?  2)  Should I make sure to buy only 5.56?

thx

5.56 has higher chamber pressure than .223, so you can go to .223 in a 5.56 chamber, but not the other way around. So feel free to shoot .223 in your 5.56 chamber to your heart's content.

5.56mm chamber = {OK} 5.56mm and .223 cal
.223cal Chamber = {OK} .223 cal {BAD} 5.56mm

great thanks