To make it more confusing I run all my 5.56 virgin Lake City brass through 223 FL die. lol. Then I trimmed the necks down to 223 minimum suggested OAL. That way I can run them and everything I own.
…after all the posts, articles, forum reads, and debates, the only thing that I’ve gotten out of the difference is the 556 military case is thicker and it has a longer case neck and can/will cause higher pressures with the same powder charges as the 223 because there’s less case capacity from 5.56’s thicker casing walls. So like always if you switch cases or if you use a 556 case work your load up from the ground up. Just remember there’s less case capacity in 556 cases so you’ll find that you’ll hit higher pressures quicker.