Rifle Scopes POA-POI at 100 yrds, but 4-5 MOA right at 400 yrds ?????

[Se7eN]

I have a Leupold VX-3 2.5-5 Ill atop my Recce clone. I have it zeroed at 100, and the rifle shoots lights out. But when I adjust for 400 yards, I'm hitting roughly 4 to 5 MOA right of POA. If I adjust for POA at 400, and come back down to 100 yards, I'm hitting 1MOA left.
Could this be a canted optic ? Or am I looking at dreaded spin-drift ? I'm shooting Black hills .223 69 Grain HPBT through a 16 inch 1/8 stainless match BBL.


Thanks in advance,

Lee.

 

[roggom]

Your base is canted to the right. I am not sure what mount or base you are using, but this happens also on bolt guns with the standard leupold base and rear windage screws.
Your scope body is canted a degree or even a few minutes to the right. Using the windage knobs only adjust the internal crosshairs, you have to move the scope body to the left somehow.

 

[bodywerks]

^^^ this. But a canted scope isn't really going to cause that much of of a shift. Its actually quite negligible so long as the scope is level when shooting.
Not accounting for spin drift alone will shift the bullet about 1-2 inches at 400yds. 5mph of wind from the left will shift it an additional 3 inches or so.
And of course, if you're not keeping your reticle plum with the earth when you shoot, any elevation you dial will also impart a windage value

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[roggom]

The grosser the cant (or better word would be yaw) the more it will be off at further distance. I guess what I am saying is the scope is not perfectly aligned with the barrel / receiver.

Anyone can simulate this at home, take a flashlight (simulating the scope), hold it to your body and point it at a spot on the wall (a sticky note or maybe a lightswitch). If you are directly in front of it and the light is perpendicular to the wall/ switch, as you walk back ward, the light will not move from that spot.

Now take a half step to the left and aim the light at the same spot. The flashlight will be angled to the right, without moving the light step backward and the light will walk to the right. (poi shift to the right)

While you are back a few steps and to the left now center the light again(simulating a re-zero) and walk forward, the light will walk to the left.

I deal with this kind of thing all the time with medical optics , its a real pain in the rear when things are not correctly square or plumb.

 

[sobrbiker883]

Leupold is OK with their scopes having some cant in them....then there's the mounting, and the level of the rifle. Wind call needs to be spot on with 223's in the breeze.

 

[Defusion]

Also, 4-5moa or 4-5" at 400?
You would need about 10mph to throw it 4-5moa, but considerably less for 4-5".
(I'm calculating with a ballistic calculator in meters and converting manually. Please correct me if I'm off somewhere)

 

[Shoalwater]

4 to 5 MOA is 16" to 20" at 400yds. And if your off that much you have more than just a little cant going on.

 

[stranded]

I'm a hunter mostly, some paper, I don't hve a need for super robust mounts. That said, I have found using an adjustable base(my case,Leopold) and laser bore sighter helpful. I center the cross hairs mechanicaly(see sticky's) then center the bore sighter in the bore. Target as far away as I can see laser. Center cross hairs Windage over laser dot. For me this method seems to put scope in line with the bore. Very little windage adjust used after out to 450yds. Usually wind at range accounts for misses. That and poor fundaments(somtimes, just mental).