I have always intermixed factory 223 and 556 with no problems. 223 has a SAMMI spec because it's a civilian cartridge. 556 uses a different standard. The pressures shown for both vary due to the different methods of taking them required by each spec. One of them has a different throat depth if I remember. There is a Wilde chamber that averages the differences between each and is said to be suitable for both. My AR Varmint gun has such a chamber. I've only shot my 1V in 223 with reloads using the receipe that came with it as a quarter will easily cover a 5 round 100 yard group. 99% of my brass is old military but reloaded in RCBS 223 dies.
