While shooting at the Whittington Center last weekend I encouuntered a situation with my LV gun that has the Brackney mounts. During the LV 100 I shot a .135 and a .162 on the first two strings. The first shot on the sighter bull on the third string was not visible. Two more shots were also ghosts. Went to the larger sighter bull and that shot resulted in a hole in the bottom of the upper box. Finally found the other three sighters that were hiding in the target number writing in the middle of the target paper. My elevation was near 12 inches too high. Went back to the lower right sighter bull and reset the elevation using the technique described by Pete. Unfortunately I ended up with a four inch penalty on that target.
Had no further issues with the mounts for the rest of the competition.
Things learned, the knobs were not loose and moved firmly. While the knobs have index marks, they are not labeled, so I took my sharpie marker and placed a small dot on the knob to mark my 100 yard zero. The 200 yard zero is about 1 1/2 index marks off from the 100 yard mark.
Now to the curious stuff. Two other shooters had the same thing happen to them. Their knobs were not loose either. Both shooters were very experienced. One shooter never found the paper. So, the questions remain, mechanical failure, user failure or third party mischief? What is the likelihood of three identical occurrences that are the result of mechanical or user failure?