I would stick with the receiver mount only. I mounted an ATI on my buttstock, and it was in the way if I have to shoot with my off hand (which could be a very real possibility). Besides, the ATI mount would only allow 4 rounds, not the five advertised. If I put all five in, it would flex outward from the stock and try to pull away from the stock. Leaving the middle round out worked O.K., but I wasn't satisfied with it.
For Christmas, my wife bought me a Choate MK6 stock for my 870. It is like a thumbhole stock, and allows carry of five extra rounds in the cut-out in the stock. Brownell's has them, approx. $73.
