optic won't tighten

Well, I got my Primary Arms Micro Dot optic in today and was all excited until I put it on the rail and realized it won't tighten down enough to prevent its sliding back and forth. Primary arms said they could send me a new base they can grind down but i'd need a "caliber" to get the measurement of my rail.. hmmmm.. no clue? Any help or should I just take it to a local gunsmith to mount it?