Originally Posted by Ceathreamhnan
What if you were to fix the gun on a rest with the scope sighted on your 25y zero, then held or clamped a laser pen-type device on the same axis as the turrets and pointed that at the 25y target, then held a card in front of the barrel so the laser pen showed the sightline on the card? The laser dot would be shifted sideways a bit but you could use parallel lines to get it measured.
I don't think that would work. If the turrets on your scope had a level top and you did manage to fix a laser pointer on top, the laser would never cross the trajectory of the pellet. (assuming your scope is mounted level and doesn't have packed mounts or adjustables).
The reason it wouldn't work is that the line of sight from your scope isn't parallel to the scope body, it's angled down by the internal lenses in the scope so that it crosses over the trajectory at some point.
The illustration I posted a few posts back has exaggerated angles on it, but you'll see what I mean if you look at the sight line coming out of the scope on the illustration.