Originally Posted by Dr_C
I have one of those little Hawke bubble levels that clamps on to the dovetail, so the gun and stock should be vertical.
I've seen others do it by eye, and the person I shoot with regularly does it empirically, shooting long and short until crossover goes away. Is there any other way?
It's plumb from the latin 'plumbum' meaning lead, for the weight on the end of the string.
I do it by eye first. I then place a target paper out at 15 yards or so. I set it up level and take a longer level and draw a perfectly vertical line with a sharpie. I shoot at the line keeping my vertical cross hair on the line and adjust until I'm hitting the line. I then run the elevation up a turn or two and shoot at the line followed by turning down the elevation a turn or two past the original setting. All the shots should be on the line. If the scope is not plumb, the groups will fall to one side with the high shots and to the other with the low shots. The hard part is remembering which way to rotate the scope to make the correction. I have found that with BKL mounts, I can just set them on a level surface and mount the scope with a small level on the turret cap. I shoot a gun (TX200) with no flat spot for a level and this method gets me very close.