I want to stretch the legs on my AR15 out to 500 yards. It is a decent quality rifle with a 16 inch barrel, 1:7 twist. I have a cheap scope on it now but want to add a Leupold or Vortex for $1000 or less. Any thoughts on appropriate magnification and objective lens size? I have a vortex pst gen II on my RPR 6.5 Creedmoor and feel like that Scope may be overkill for an AR15 given unknown accuracy potential at 500 yards.