Re: why .308?
True. IIRC, the NATO chambering is throated for something like 150gr bullets, and most civilian .308s are throated to chamber a wider variety of bullet weights. It is still cut w/ the same *body* reamer, for all intents and purposes, AFAIK.
Saying that because one gun is throated short for one bullet makes it a 7.62x51, while another is throated longer makes it a .308, when they have the same case body doesn't make a whole heck of a lot of sense to me. So we have the .223 Remington, as another 'case' in point (pun intended). Chambered short for 40-55 gr bullets, w/ a slower twist, it is a .223 Rem. Crank the twist up to something like 1-8" or 1-9" and feed it 62gr SS109 bullets, and now it is magically a 5.56x45 NATO and no longer a .223 Rem? So what does it become when you crank the twist up to 1-7" and throat it for 90gr JLK bullets? Guess what? It is still a .223 Remington!!
For most purposes, .308 Win = 7.62x51NATO, and .223 Rem = 5.56x45NATO. The throats are cut slightly differently, but the case bodies, AFAIK, are different. One is the 'old-fashioned' U.S. designation i.e. .308 or .223, and the other is an international metric designation i.e. 7.62 or 5.56mm.