Welcome, the builders on this forum will point you in the right direction. If you don't shoot more than 175 gr, the 11.25 twist will do fine.Any more take DRD's advice. Actually this is a 308AR build. AR10 is proprietary Armalite. For good values on optics, check out Primary Arms. Also the Nikon Monarch line. A 4x16 would work, actually more optic than you need at those ranges.