A locking mechanism, such as thumbscrews 130A-B or the like, extends through each one of the slots 116A-B. This permits this side arms 128A-B to be secured with respect to the radial arms 124A-B. This, in turn, permits the reticles 112A-B to be securely positioned such that the sighting line 114 intersects the trajectory 106 at an appropriate point along the trajectory 106 that corresponds to the