That Winchester warning only applies to heavier bullets because the
ogive is
blunter and the bullet is longer.

That's only the Winchester statement. They are not the only ones who
speak on the topic. Read the SAAMI website. Anybody that tells you
there is no difference between a 223 and a 5.56 doesn't know what they
are talking about. Once again ...... SAAMI states that it is an unsafe
practice to shoot 5.56 in a gun that is only designes to shoot 223.
They are the experts.