Colt AR15 HBAR Question

[firstrock]

Folks

I am not new to AR's but was wondering about my AR. The receiver is stamped .223 but the bbl is stamped 5.56 NATO. So, I would assume that since the bbl/chamber is for 5.56 the I can run both 5.56 & .223 in this gun. I understand the difference between the two rounds but am a little confused on why the receiver isn't stamped 5.56. I would assume this is just a manufacturing thing as many lowers are used for different uppers??

As a side note, this was my Father's gun, purchased for me when I joined the Army. I have shot this rifle but not sighted it in for me yet. I plan on getting it to the range soon and would like to see how far I can accurately shoot at long range. I have plenty of experience using iron sights out to about 500m but not much past that. I also have a Winchester Mdl 70 in .223 single shot (Weaver scope) that I may use for longer range ('yotes) but have not shot or sighted it in yet either.

I'd like to get started in CMP and Appleseed this coming year and want to set the guns up this winter.