Good answer for the purpose of the rifle - usually catches people off guard when you ask them, then you get an answer back that they want it to do everything... You thought out the purpose for the gun.
If you'll never go over 600 yards, I'd just run a 16" barrel with midlength gas. No need for any more barrel than that. If you'll go a little longer, like out to 850, maybe an 18" barrel. FOr whatever distance you'll shoot, there's no reason at all to go over an 18" barrel - not for what you'll use the gun for. My $0.02 on it.