We've got a couple gun shows coming up and i'm looking to get an AR. In preparation, i'm counting on the seller not knowing the answers to any of my questions, and would like to be able to find out for my self. So...is there a way i can tell by looking at the gun whether the barrel is chrome lined or not? How about what the rifling twist is? What about carbine vs mid length gas system? I may come up with a couple more, but these are the ones on my mind right now.