The typical 308 AR isn't piston driven (though a select few are), rather are Direct Impingement systems and thus aren't as adversely affected by overgassing. Semi-autos like the M1 and it's variants can be sensitive to the issues you are talking about, but the average large-frame AR probably won't be unless it's a piston-driven system. I'd venture to say that outside of shooting suppressed, off-the-shelf ammo won't be a concern with regard to a DI gas system.