Originally Posted by Monty
If I've interpreted the question correctly, Blacklab is trying to establish his scope height (actually it's the crosshair height which is different) above the bore at the end of the barrel, not at the end of the objective, which again is a different measurement.
One method is to put tin foil (or anything completely opaque) over the objective and make a really small hole in the centre. I use the end of a biro to punch a hole 1-2mm diameter. This creates a lens akin to a pinhole camera and increases the depth of field by a huge factor. You then put a piece of paper with a black dot at the end of the barrel and if all is well you should be able to see the dot through the scope as long as it's bright enough (hence the torch) and you've set the parallax to the closest distance the scope will allow. Once you've centred the crosshair on the dot you fire a pellet which will punch a hole in the paper. You then measure CTC between the dot and hole and you have your scope height which is a critical measurement for all ballistic calculators.
Forgot to add that the hole needs to be pretty bang on the centre of the foil. I put the foil on and crimp it back so I have a clear outline of the end of the scope. Remove the foil gently and fold it in half to get 2 perfect semi circles and then in half again. I now know where the centre is. Punch the hole and carefully put it back on in the same place as it was using the outline in the foil.
Thanks, I always use the measure scope/barrel diameter then take one from the other routine, but I can see the benefit of using the cross-hairs though to be perfectly accurate I guess you would have to optically centre the scope first.