Scope Cant

I ran into a weird problem with my Hatsan a while ago.... I took my usual care mounting the scope, including making sure the crosshairs were vertical, sighted the gun in at 40 yards, and then decided to see how it would do at longer ranges.... I was very happy with the groups, but the further I went out, the further the groups were to the right, and there was basically no wind.... At 100 yards, the group was centered almost 2" to the right, let's call is 1/2 a MilDot.... Anyway, I resighted the gun at 100 yards for windage, using the 4th MilDot down which was the perfect elevation for that distance.... When I went back to 40 yards, the gun now shot 1/2 MilDot to the left.... I chalked this up to some weird spiralling action of the pellet, although the groups were tight enough I really doubted that could be taking place....

This morning I woke up thinking about what had happened, and it occured to me that if the scope was twisted in the rings slightly, because I was using the MilDots for elevation but the vertical crosshair for windage, if the vertical crosshair didn't line up with the barrel I could get exactly what I had experienced.... I drew out a diagram to confirm this....

http://i378.photobucket.com/albums/o...ps98a59efa.jpg

The blue line on the reticle is what the trajectory looked like relative to the crosshairs when sighted at 40 yards.... It was 1/4 MilDot to the right at 76 yards (2 Dots down), and 1/2 MilDot to the right at 100 yards (4 Dots down).... After I resighted to correct the windage at 100 yards (but still using the 4th MilDot down) the POI was now 1/4 MilDot left at 76 yards and 1/2 MilDot left at 40 yards as shown by the red line.... Now imagine what would happen if you twisted the scope in the rings a few degrees counterclockwise, so that they lined up with the POI at 40 yards AND at 100 yards.... The problem should go away.... After figuring this out I checked my scope, and sure enough, the vertical crosshair wasn't perfectly lined up with the bore, and it was out to the left, as predicted by the drawing....

I haven't had a chance to correct it yet and prove this, but it certainly seems logical.... An error of 1/2 MilDot in a drop of 4 MilDots is only 7 degrees of twist in the scope....

Bob

Here is a tool designed to look aftter this problem....

http://www.projectsavage.com/?q=cont...cle-instrument

Nice to know it actually IS a problem, not a figment of my imagination, and that there is a relatively easy way around it when mounting your scope....

Bob

Well I'll be....isn't that a nifty little rig!

That would certainly explain why my .177 '78 shoots about 3/4" to the right at 55 yards (20 yard zero). Consistent, but always to the right. I, like you, just assumed it was the pellet "doing its thing" and adjusted my POA to compensate. Guess I have some work to do!

I think that most scope mounts are level when properly mounted however it might be possible the the reticule isn't perfectly level, the the scope tube isn't mounted level. That's where I find a Bushnell boresighter useful.

Boresighting won't cure this problem.... If an extension of the vertical crosshair doesn't intersect the center of the bore, this can occur when you use holdover....

Bob