Re: Ammo Carrier
A sniper's hide member posted a self made setup just like what you are describing a while back. I can't find the post, but what he did was use an extra scope ring and mounted it to the side of his scope with a rectangular piece of metal attached to the bottom of the scope ring. He then attached a velcro strip onto the metal piece and then a redtac ammo card.
Another option, for something that you can order ready made, is S.O.Tech rifle bullet foregrip pouch...which sounds like what you are describing.