Need a little help with interpreting scope data

[DZelenka]

In the past, I have mounted scopes and just leveled the crosshairs holding the rifle offhand. I do a pretty good job as will be seen below. Typically, I zero my deer hunting rifles 2" - 2.25" high at 100 yards which is dead on around 225 and 6" low at 300 (.280 Rem 140 BT). I have used it out to 325 yards and feel comfortable shooting a bit farther than that using holdover. I shoot highpower and feel very confident about 1st round hits to at least 600 yards with adjstible turrets so I decided to rig my rifle better for the task. I received a 3.5-10x40 Leupold CDS with a plex crosshair as a gift and sent it back to Leupold to install a CDS windage turret. After reading about how some of you guys check the installation of your scopes, I tried the following experiment:

I used a blank piece of paper at 100 yards. Using a level, I drew a vertical line 18" long. I thin drew a horizontal line to form an upside down "T". I shot my rifle on my 100 yard zero (needed a bit windage to get it centered). To save a bit of time and most importantly barrel heat, when it looked centered, I added 15 MOA and fired 2 more shots. As you can see from the attached picture, the center of my 100 yard group was about .3" left of centerline and the +15 MOA group was on the centerline. If you drew a parallel vertical line through the lower group, the center of the upper group is .2" from the parallel line.

Here are my questions:

1) Is this deviation within acceptable tolerances for shooting out to 600 yards? Or should I try to turn the scope in the rings to correct it?

2) I have been experiencing an odd phenomenon. When My scope is zeroed for 100 yards, I get some horizontal (still well under MOA). When I add elevation 9 - 15 MOA, I always end up with all shots touching. One day I even did this alternating shots between zero and +9 MOA. Weird. Anyone care to comment?

Rifle is a McMillan Signature. 24" #3.5 barrel (Rem mag profile) with Burris Weaver style mounts and Burris low 1" Zee rings.

Any comments appreciated.

Dan

PS Those are two 2 shot groups with a 1/4 MOA right wind correction between them.

 

[BuckSnort]

Are you using a bubble level when shooting?

 

[DZelenka]

Are you using a bubble level when shooting?

No I am not, but the horizontal and vertical crosshairs were aligned with the horizontal and vertical lines on the target which had been drawn with a level. That should make the scope level when fired.

 

[BuckSnort]

No I am not, but the horizontal and vertical crosshairs were aligned with the horizontal and vertical lines on the target which had been drawn with a level. That should make the scope level when fired.

Scope would have been leveled yes but that doesn't mean the scope is leveled with the rifle... How did you mount the scope?

 

[DZelenka]

Scope would have been leveled yes but that doesn't mean the scope is leveled with the rifle... How did you mount the scope?

I just eyeballed it. 35 years of doing that puts me pretty close.

The test that I did is to determine the levelness of the scope. If the scope is not level to the rifle, when you raise the elevation 15 MOA, if the scope isn't level to the rifle, it won't track in a straight vertical direction. My question is more about whether I need to turn the scope clockwise slightly to remove the slight horizontal tracking to the right when I raised the elevation. Perfect would be great, but I am trying to determine what experienced long range hunters find acceptable.

 

[BuckSnort]

Unless you mount it to the rifle compleately level you're gonna be chasing you're tail trying to get things perfect..

 

[Wingnut]

For 15 MOA of adjustment that is pretty good. It would be a pretty small adjustment to try and get that angle of degree out. You could try but might not make it back to where you are now. Now the real test shooting it at 600 yards. This will show you your spin drift and a new factor to adjust for. As for the dispersion left to right at your zero. Does it do it through the whole adjustment? At 8 MOA is it half as big of group? If it were bedding or an issue with the rifle you would see it throughout the adjustment. It might be an issue with the scopes erector setup.

 

[DZelenka]

For 15 MOA of adjustment that is pretty good. It would be a pretty small adjustment to try and get that angle of degree out. You could try but might not make it back to where you are now. Now the real test shooting it at 600 yards. This will show you your spin drift and a new factor to adjust for. As for the dispersion left to right at your zero. Does it do it through the whole adjustment? At 8 MOA is it half as big of group? If it were bedding or an issue with the rifle you would see it throughout the adjustment. It might be an issue with the scopes erector setup.

At 600, it seems to be about 1 MOA right of my wind call. 1/2 MOA of that should be spin drift so there is somewhere between 1/4 and 1/2 MOA misalignment. You answered what I was asking although I would like a few more responses. Getting it much better than I have it may be chasing my tail.

It isn't a rifle issue, it seems to be a scope issue. I don't get any of the horizontal dispersion when I add elevation. The last 3 shot group I shot with 9 MOA on was slightly over 1/4". I thought about getting a 10 MOA rail, but it would raise my scope which I don't want to do. Plus, I may have to screw with the crosshair alignment again.

Dan

 

[paphil]

Dan, What you have is an internal scope problem. Not really a problem in your scope because the difference between your vertical reticle and the scope adjusting mechanism is so small. From what your picture shows, your adjuster in the scope leans slightly right. Many scopes have this problem, some lean or cant as much as 4 degrees, Yours looks to be about 1.5 degrees. I also once thought that rotating the scope in the rings would fix the problem ,but doing the same T test that you did proved me wrong. I found that the only way to correct was to use a scope level canted slightly in the opposite direction. The goal being to shoot with the adjusting mechanism perfectly vertical even though the reticle will be canted slightly left in your case. I go one step further and cant the scope (and adjusting mechanism) an additional 3 degrees left and install the scope level in level position to get the gun to shoot one minute left when turned up 22 MOA for 1000 yards on the T test. This eliminates spin drift from the equation, but you have to rely on the scope level every shot. I find that the only time I notice the reticle being out of square is when shooting targets that have been set with a level.