First off, 308 AR's are different the AR15's. The main differences are obviously size and weight and one that's not obvious is that they are more critical to shoot accurately than bolt guns. Any flaw in technique or follow through will show up with a 308 gas gun that won't in a bolt gun. Shorter barrels tend to show less issues with bad technique than longer barrels.
Second, There is no way I would choose to shoot a 20in barreled AR10/SR25 unless it was a dedicated match rifle. A 18-20in barreled 308 AR with a suppressor is just stupid ungainly. Suppressed 308 gas guns that are not dedicate match rifles, are best with a 16in barrel... or shorter. They can still be shot to past 800 yards, yet are somewhat manageable. They're still pigs, but manageable pigs.
