The most recent issue of Small Arms Review has a nice write up of Ruger's direct drive .308. This article, a giant article on the PIAT and making your own cannon (which I suddenly want to do), make it worth the purchase price.
That article would be incorrect. The SR-762 accepts SR-25 pattern mags (i.e. KAC, DPMS, Magpul, Promag), which has become the industry standard. The Armalite AR-10 is its own beast, and uses a mag design derived from the M-14. Even Armalite has figured out the train has left the station without them, and has introduced a couple AR-10 models that will accept PMAGs and such.