Wow, so many questions. I would stay with 7.62x51mm, I have sent >16K rounds down range through my NM M1A. 90% of it was military surplus ammunition from around the world...
What I have personally fired through my NM... http://m14forum.com/ammunition/23464-mil-surplus-ammunition-manufactures-7-62x51mm.html

What I have personally fired through my NM... http://m14forum.com/ammunition/23464-mil-surplus-ammunition-manufactures-7-62x51mm.html
