A reader, Dan, fully versed in scope issues, reports he once spoke to a Leupold tech and learned a fully two-thirds of the scopes returned to Leupold for “repair” aren’t broken. Leupold generally just gives the scope a good inspection, charts the erector movement, and returns the scope.
What causes someone to think a scope “failed” when it really hadn’t?
The most common reason: a mounting problem.
If you mount a scope on a base which isn’t aligned closely to the barrel’s bore (that’s bore, not necessarily barrel, since some bores aren’t drilled straight through the barrel) you may have a hard time zeroing the scope at particular distances.
If the base is slanted up or down, as opposed to being parallel with the bore, you’ll need to dial in quite a bit of elevation correction to get on target. Normally, this won’t be much of a problem unless you run out of “come ups” for that 600 yard target you’re trying to hit.
Or, if the scope base is off to the left/right with relation to the bore, you’ll need to dial in quite a bit of windage to get zeroed. Here’s what can then happen:
In the top image, the green circle represents the erector inside the scope tube. The erector is relatively centered, with plenty of room to move up/down/right/left. This is how you hope your erector will align when you zero your rifle.
In the middle image, the erector has been dialed over to one side. This may be to compensate for a base/ring system that isn’t in line with the rifle’s bore. While you can zero the scope for closer ranges, you won’t be able to dial the elevation up to reach longer ranges because, as you can see in the diagram, the erector will hit the inside of the scope tube at about 2 o’clock (or 4 o’clock if it is moving down). In this scenario the scope will zero at 100 to 200 yards, not at 300 plus yards.
The bottom image represents a “dialed up” erector; it’s at its upward limit of movement. If you were to try to dial in 15 MOA of windage at this high elevation setting you might get half of that. Then the erector would hit the inside of the scope tube and stop.
Larger scope tubes don’t really have any light transmission advantage, but they have a significant advantage for internal adjustment capacity. A 30mm scope tube mounted on a “crooked” base and ring set will still dial in, and likely work fine as far as “come ups” too. Therefore, one might replace a 1″ tube scope with a 30mm scope and find that a tracking problem has gone away. The 1″ scope will be blamed, but a faulty mount may be the true culprit.
Remember, that problem scope might not be the issue. In most cases, where a well-made scope won’t dial the correct elevation, it really isn’t a scope issue at all, but a mounting issue.