If I'm understanding this right, it's basically an AR15 with an extended magwell to accommodate the .308 cartridge instead of being an AR10. I think POF or somebody already did this but you don't hear anything about it anywhere. Lightweight semi-auto .308 for $1200 MSRP, might make a good semi-auto hunting rifle that takes common mags but maybe not a full-on battle rifle.
https://ruger.com/products/sfar/models.html