Vertical Alignment

[woods]

I have a Kahles MultiZero on a 300 win mag. Using a ballistics program to calculate trajectory for a 200 gr Accubond at 2900 fps I put some of those 1" orange dots at the projected impact points for 100, 200, 300, 400 and 500 yards (0, +1.5", +3.7", +6.3" and +9.1") so I could preset the marks on the turret before going to shoot at the longe ranges.

With each successive elevation mark, the POI drifted to the right so that by the 500 yard mark the POI was approx 3" to the right.

My question is: Is this a sign that the scope needs to be rotated slightly in order to align the vertical axis along which the scope adjustments move to the true axis of the scope and bore? In other words, when clicking on the upper turret the center of the reticle is not moving exactly vertical.

Hard to explain. Do you long range shooters who use turret adjusments ever run across this problem or do you ever click up say 30 clicks and check to see if your POI is in vertical alignment with the lower group at your 100 yard zero?

 

[ST42]

Woods,

First let me state that I am just a beginner when it comes to shooting past 500 yards. But it seems to me that 3" of drift at 500 yards is less than MOA. If you can rule out hold and check weld errors along with any wind component drift. I would start to consider the possibility of drift caused by the spin of the bullet. I seem to remember reading something about this on another forum, but I can't remember what they called it.
Of course it could be that your scope isn't aligned vertically to your bore...... Not much help, but I know I'll be interested in finding out the answer too.

Shawn

 

[woods]

Thanks ST

Perhaps I miscommunicated. The drift is 3" on the 100 yard target after clicking up 28 clicks to the 500 yard mark.

Wish you could post pictures here.

 

[ICANHITHIMMAN]

Check your scope

I got a little tip for you on scope mounting. you need to shoot a box at 100 yards this will help determine if your rectical is canted and how good you scope tracks.

Zero your rifle at 100yds

Use the same aiming point every time

Shoot 1 round and then turn your windage dial to the right 3 inches shoot another round then dial up 3 inches shoot 1 round then dial left 3 inches shoot 1 round and the down 3 inches your last shot should be on top of your first. This is a box it should be squar. If it help's try using a target with grip lines on it, and rember to use the same point of aim every time

 

[ST42]

Woods,

Actually, I just need to read the question...

But for what it is worth, I found the link to the info I mentioned. They call it ..."Spin Drift". Go Figure.

Spin Drift??? - Sniper's Hide Forums

 

[woods]

Hey ST

Thanks for the link and I found this in CatShooter's post

"But modern scopes with internal adjustments do not... and the cross hairs in scopes can be 2 to 4 degrees out of plumb to the spindle faces, so you can't use the cross hairs to set up the bubble, (which is what EVERYBODY does).

If the bubble is not true to the spindle faces, then when you crank in elevation, you also crank in a little "windage" because the hairs also crab to one side or the other."

I don't have a bubble but I do believe that cant is my problem. I will rotate the scope and let you know.

Let me see if I can post a picture

Hey, it worked! I had already shot the 400 yard mark on another target, but you can see where the 2 shots at 200 were a little right, at the 300 mark they were noticeably to the right and at the 500 yard mark even further.

Be a great way to get the scope reticle level - shoot at your 100 yard zero, click 30 up and shoot, see if the POI of the 2 groups were vertically aligned.

 

[Jon A]

First, did you set up the target with a level to make sure it was straight? Second, when you shot at the target, did the vertical crosshair go through all the dots ensuring it was square with the target?

If the answer to both is yes, and it'll do that repeatably, your reticle is canted--a lot! Send it in--they will fix it.

 

[davewilson]

actually, your reticle may not be canted at all. don't know about your scope, but most have a circular tube that holds the crosshair. when making adjustments, you are simply pushing on the outside of this tube either on the side or the top. the farther you are away from the center of adjustment, you will actually move the horizontal when changing the verticle. again, the farther away from center you are, the more it will affect the other adjustment.

 

[woods]

Hmmmm, food for thought.

I scanned a sketch in to show why I think that if may be the scope is rotated

For example if the scope were rotated a little clockwise, clicking elevation on the top turret would move the internals along axis A. When raising the barrel for the new aiming point, it would move up to point G and then over to point H which would throw the POI along the axis D.

Possible?