5.56 NATO and .223 Remington cartridges are dimensionally identical.Now I'm going to confuse the issue a bit; your Colt HBAR should, as noted, be chambered in 5.56. There are slight dimensional differences between the military 5.56 round, and commercial .223. You can swap them one way, but not always the other. Someone who's certain about it will hopefully chime in, and I will refrain from making an incorrect statement about which is which.
Edit: Oooops, 5pins got it, already.