My impression is that the two main differences between .223 and 5.56 are: 1) the throat on the 5.56 is a bit longer, and 2) the 5.56 produces about a 10,000 psi higher chamber pressure than the .223.
Having said that, quite a few manufacturers seem to have beefed up their chambers for 5.56, even though they are “nominally” chambered for .223. I suppose the longer throat would make a .223 fired from a 5.56 somewhat less accurate, but I’m not an expert.
Of course, you should check with the specific manufacturer, since AR-15’s etc seem to be multiplying like rabbits (NOT apologies to the likes of Carter and Kerry!).
I also think (based on “some” reading) that the 7.62 and the .308 are virtually identical.
I corrected that brainfart in a follow-up post, as the shorter leade on the .223 causes the increase.
Yes, there is also the case wall thickness on the military brass being thicker, reducing the interior area, giving higher pressures with similar loads. Something to watch out for if reloading surplus brass.
I have not messed with the .308/7.62 in a technical way and would not comment off the top of my head on it.
I just built a varminter AR from a bbl blank up for myself. It shoots sweet! The hard part will be keeping the wife from claiming it.