So what you are looking for is a semiauto .308, the shorter the better, capable of shooting an 8" group at 600 yards, and which can switch back and forth from open sights to optics?
Everything you want is a compromise on another specification. Shorter means less accurate. Being able to use open sights without removing the scope means the scope sits higher than is optimal.
I would suggest a CETME Sporter in .308 and a scoped bolt action hunting rifle with a heavy barrel in the same caliber.
I don't know about the CETME accuracy wise, but the HK 91/G-3 is every bit as accurate as a bolt action rifle and you don't have to carry two rifles! The same go's for the SA58, they're accurate as hell.
I think the CETME is accurate enough to be lethal inside 200 yards or so. The bolt gun is for the 600 yard shots.
I think a lot of people want a short, handy combat rifle that can do double duty as a sniper rifle. It is difficult to work both ends of the scale like that. Since he said "Shortest OAL without the hassle of SBR" he is looking for something with an 18" bbl that will reach out almost 1/3 of a mile.
I have read nothing but praise for the HK91 and the SA58. But both rifles, in their shorter formats, would take some serious skill to put shots on an 8" target at 600 yards.