My STHF, TEOTWAWKI rifle is a CETME or H&K91 or an FN FAL (18" barrel) or a standard M-1A or even an M-1 Garand. I want a rifle that will kill large game as well as people. I want to put them down with as few rounds as possible. I want to hit out to 600 meters and hit with authority at that distance. I do not plan to run around with 1000 rounds of ammo when I'm doing other things to stay alive. I'm not assaulting Son Tay, I'm trying to keep my ass alive when I cannot depend on others, survival is like a recon mission, I want to see the other guy first and avoid him I only want to shoot if I have to and I want to get the hell out or put the other guy down quick, noise draws attention and attention is the last thing I want or need.
The SOCOM 16 is a special purpose rifle not a main battle rifle, the short barrel makes it lighter and handier but increases noise and flash while reducing velocity, hitting power and range. I want at least an 18" barrel on a 7.62NATO rifle to give me the best performance across the whole spectrum of engagements. Yes a long barrel and a heavy rifle is a PITA but then the chips are down you want every advantage you can get. For those who will inevitably pipe up about how they don't need 600 yard range in their urban environment, I live 42 years in Detroit and I can tell you that there are 600 yard and longer shots around every corner and down the street and across the parking lot and everywhere else you want to look. Besides even if you never need the range you might well need the power. Brick buildings, telephone poles, hedges, houses, cars and all kinds of other cover can be turned into concealment if you have enough gun. I want to hit the guy I'm shooting at even if he hides behind a tree, a 7.62 can do that, .223's mostly won't.
For the record though, If I could get a 6.5 Grendel in a decent platform and 123 and144 grain rounds at a decent price I would trade in my 7.62's in a heart-beat, but so far that hasn't happened. I drool for a Beretta ARX-160 in 6.5 Grendel with an ACOG optic, it's not the best design I've ever seen, but it is close.