As I pointed out in your other post. The benefit of going 6.8 SPC is it fits in a 5.56 size frame gun and delivers slightly more performance. Putting it in the .308 size mag may give you a longer COAL, but you would have to be using an AR10/.308 upper too. As the magazine wouldn't feed into a shorter and smaller 5.56 upper on the big lower. If you want more performance you just need to step up to the .308 size family of cartridges. There are plenty of lightweight .308 rifles available now days.
The 6.8 still has its benefits. Fitting in a 5.56 size gun, more power, lighter ammo, etc...
YHM has a new 6.8 SPC build package that is pretty attractive. Aside from the muzzle thread pitch it's a good barrel. It should be 5/8-24 ideally. Especially if you want to run a suppressor on it. 6.8 suppresses pretty well actually.
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