I am going to build my first ar.  I want to have a 16" barrel car setup with 6 position stock.  I would rather have a mid length hand rail or grips up front.  Is this a completely different barrel from a carbine barrel?  Please let me know what I need to know, Thanks

You did not mention what you use the rifle for but...  The barrel would be the same, but the gas block would need to be different to go mid length (a shaved gas block).  I went with a mid length setup on my 16" upper and eventually took it off because it weighed too much (YHM spectre forend).  I have since used a carbine length aluminum free float tube with some rails attatched.  I would think long and hard about how much weight you are willing to toss around and exactly how much stuff you really need to hand from the rifle.  I would go with the lightest possible railed forend I could if I were you, in other words, acarbine length rail system.  If all you need is a grip, then I would not go too crazy with an expensive rail system.

Thanks, I was considering the mid length to increase sight radius and I'm tall so I was looking for a bit more forward reach.  The only thing I want on the front is a folding handle off the bottom of the grip.  It sounds like it would just be easier to stay with the carbine length for balance and weight.