h,
Welcome to the gab fest.
Your first question:
Functionally the brass and chambering for both the .223 Rem and the 5.56 NATO can be considered to be the same. If you look at the actual drawings, they are different, but not much. There is a difference in the throating between the two at the chamber barrel point. Much has been said about this, both good and bad, true and false. What counts is that the loader should start at a beginning load that is known to be safe and slowly increase the powder charge until the loader reaches the desired loading. Loading books/manuals/data will have a listed max load. This must be considered the hottest loading, but only after the loader slowly works up to it, if needed. That max is for your safety.
So, yes, start with the listed 'starting' load, for either the .223 or the 5.56.
Your next question:
I have found with testing that two or more weapons that are the same model, chambering and barrel length will all perform at different levels with the same ammunition loading. The 'book' loadings are to give a general reference point with changing components. Your/other weapons will have similar results but not the same.
And to your apology for a long post..... Are you trying to tell me something
Load with care and enjoy.
OSOK