I ordered the complete Bolt and Bolt Carrier Group for my AR from Bushmaster. I have a question that I probably should have asked before ordering. There is the option to order it for the AR-15 or M16. I ordered the AR-15 model without really thinking about it. What is the difference between the two? I was under the impression that the M16 was basically just a more customized AR-15. Also, while I am thinking about it, would it be better to order a barrel chambered in .223 or 5.56 NATO? The one I have picked out from YHM is 5.56 NATO, but would it be better to find one in .223? Is one type of ammo more expensive or harder to find than the other (I know most ammo is getting pretty scarce right now)? Thanks for all the help. Yall are the best.