Oh please, US would push back against UK's .280 British small arms cartridge[0] for NATO harmonization, despite there was already ample testing shown that the more powerful 7.62mm cartridge was nigh-uncontrollable in automatic fire. Only for the Vietnam war to demonstrate the deficiencies of 7.62NATO, and then US and NATO adopted 5.56mm, proving the initial UK testing right.
If the US can make all its members adopt a small-arms cartridge despite it not being particularly ideal, the separation between the US and NATO is merely a facade.
0: https://en.wikipedia.org/wiki/7.62%C3%9751mm_NATO#T65_series...
