I am looking at building an AR 308. I want to do some long range shooting. What lowers are avalable? The ones that use G3 mags, do they stay open after the last shot? Do the rest of them use converted M14 mags? Are the uppers interchangable other that the G3 converted ones?
Check out the FAQ at the top of this forum. It has all the info you need on interchangeability. I think the only one that uses the G3 mags was a short run by CMMG. I dont think they make them anymore.
The Armalite AR-10 is the only one that uses the modified M14 magazine. As well as the other variants who use the Armalite as a base, like Noveske and GAP did a while ago.
Then you have the SR25 pattern which uses the 1950s era AR10 magazine, or waffle mag.
Check out the FAQ. TaylorWSO goes over pretty much everything you need to know.
If I'm not mistaken, the CMMG G3 mag lower also required some minor milling of the upper to clear the mag lips.