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rsterne · Moderator

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....

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

AirGunEric · Site Admin

Makes sense- and also might be a place where a laser bore-sight might be useful to cut down on sight-in time (assuming of course it can be centered in the barrel properly) or a level matched to the barrel and scope adjusted appropriately. Of course, just shooting it after making adjustments would be the simplest...

rsterne · Moderator

I'm not sure how a laser bore-sighter would help, as this problem only exists when you are using the MilDots for holdover to compensate for the trajectory.... A boresighter is only used to set up the Crosshairs and the beam from the Laser doesn't drop with distance....

Now if you had a laser that "vibrated" to paint a vertical line on the target, and rotated it and the scope so that the vertical crosshair and the laser line were PARALLEL then you might have something.... There you go, Eric, a new product, to address a problem we didn't know we had!....

Bob

Slavia · Moderator

They have laser line levels. You would probably have to set it up so the bore sight laser dot was right on top of the vertical laser line (on the target) and then adjust the reticle for parallel.

rsterne · Moderator

I had to think about this for a day or two before responding.... Certainly, lining the horizontal crosshair up with the line from a laser level makes great sense.... I was wondering how that relates that to the cant of the gun relative to that, however.... I mean you could have the scope level but the gun tilted over....

Perhaps if you optically centered the scope and then rotated it in the rings until the vertical crosshair lined up with the dot from the boresighter at the same time as the horizontal crosshair was parallel to the level line?.... The problem I can see is that if the rings are off center relative to the boreline, you could end up with the rifle canted over somewhat when the scope is level.... Rings with an external windage adjustment would be the cure I guess....

Bob

rsterne · Moderator

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

http://www.projectsavage.com/?q=content/exd-engineering-vertical-reticle-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

Slavia · Moderator

Thanks for posting that. The tool is fairly simple, and the guy's explanation is very clear - especially the part about the difference between the vertical reticle and the turret tracking line.

Slavia · Moderator

This thread is five months old, but it has taken me this long to think through the gadget that rsterne mentioned. What I was working on was how to make a similar device with the tools at my disposal (no lathe or end mill here). The parts are all from Lowe's, a home improvement store. I used flathead screws so the countersink taper would self-center in the strap holes for less "slop." I know it looks like an Erector Set nightmare, but it seems to work. This is what I came up with:

The purpose of this whole exercise is to place the scope's axis directly above and in the same plane as the bore axis. (Re-read rsterne's post at the beginning of this thread.) Many times people do this by optically centering the scope and pointing the vertical reticle at some central feature on the gun - which seems subjective to me.

This is a HW97, and I couldn't figure out a way to actually measure to the barrel. Instead I used the sliding cylinder, which hopefully is concentric with the main tube, which hopefully is vertical to the bore. The tool rests on the cylinder (two points) and on the top of the scope's objective bell (another two points). It is adjusted by squeezing the parallelogram arrangement so all four points touch without any "wiggle" or play. The rubber band that holds it on the gun is cut from a bicycle inner tube. Here's what it looks like in place:

In addition to the tool I have a reference line just behind my 10m basement target area. It's a galvanized shelf track that I have checked for straightness and plumb. A plumb line would work as well, but the metal strip is nice because it shines when illuminated by a halogen lamp. When the vertical plates of the tool or the scope reticle are aligned with it, a little twitch will make the edge all shiny or all covered. With slight misalignment you see "shiny" at one end and not the other. Here are the steps I took:

1. Using the "V" block method I found the "optical" center of the scope. To me, "optical center" refers to a quality of the grind of the lens. This could more appropriately be called "mechanical center," with the reticle aligned with the physical center axis of the scope tube. However, "optical center" is what you see being used, so let's go with it. The scope then goes back in the mounts.

2. I adjusted the scope bubble so it indicated level when the reticle was aligned with the plumb line.

3. I moved the scope fore and aft for proper eye relief.

4. With slight movement from my normal cheek weld on the stock, I was able to sight the reference line with both the reticle inside the scope and also through the vertical straps of the tool outside. I'm assuming the mounts were centered in the dovetail, because the center of the crosshair was on the reference line with the line and tool in agreement. After that it was a matter of rotating the scope in the mounts until tool, vertical reticle, and reference line were all parallel.

5. Just to double check, I used another level on the tool, the scope's elevation turret cap, and a flat spot on the stock just behind the main tube. Those places and the scope's bubble were all in agreement:

Comments, please - don't be gentle; I can take it.

rsterne · Moderator

Should work perfectly.... I wish I still had my Mechano set....

Bob

rsterne · Moderator

I read an easy way to get the vertical crosshair lined up with the bore....You set up a mirror at 5-10 yards away, set the scope to maximum magnification, and adjust the range on the AO to twice the distance to the mirror, so that the muzzle and the scope objective are in clear focus when you aim at the mirror (gun unloaded, of course).... If the vertical crosshair on the reticle doesn't line up with both the center of the objective and the center of the barrel, twist the scope in the rings until it does.... Once you achieve that, carefully tighten the rings, checking to make sure that the reticle stays lined up with the center of the objective and bore in the mirror.... That should put you close enough to eliminate any cant of the scope relative to the bore....

In theory it works better if you optically center the scope first, but in fact I think it might be better to boresight the scope first, as what you really want is to have the reticle in the windage position it will end up when the gun is sighted in.... It is still critical to hold the reticle level when shooting, or the cant of the entire gun will throw the POI off to the side whenever you are using holdover.... I still have yet to try this to correct the error in my Hatsan, but I can tell you that when I view the image of the scope and barrel in the mirror, it is off quite a bit, so I can well imagine this will be a major improvement.... I intend to use this method from now on to align the reticle with the bore....

Bob

Slavia · Moderator

I have bubble levels on several guns (and one pistol), and I feel that they do matter. That said, they aren't a magic guarantee for accuracy. I don't feel they make much difference at ranges inside 25 yards. Also, it's easy to become "a slave to the bubble." Check it once or twice and then pay attention to the sight picture, hold, trigger squeeze, breathing, etc.

I agree that if you're compensating for shooting at different ranges, then it is important to have the vertical reticle both plumb and projecting through the bore axis. On the other hand, if you mostly shoot at a fixed range then it doesn't matter so much. Say you always shoot at that one spot in the backyard - zero it, hold it the same each time, and you're good. Same for the bubble - it doesn't have to be "level." Just shoot with it in the same spot each time.

rsterne · Moderator

Within reason, that's correct.... Hold the gun on it's side (gangsta style) and you may find not so much.... If the gun is on it's left side, at the zero range, the bullet will hit "high" (because it isn't dropping relative to the "vertical" crosshair) and "left" because it is dropping relative to the "horizontal" crosshair.... (where the vertical crosshair is laying on it's side and the horizontal crosshair is vertical, if you get my drift)....

If you look at "scope cant" as whether the reticle is aligned with the bore C/L and "gun cant" as whether or not the reticle (and the gun) is vertical, then scope cant doesn't matter if you always shoot at one range, but gun cant still matters.... but not for small angles of error....

Bob