Left @ 300 yds - Right @ 1000 yds? WTH?

[Pdvdh]

Two different rifles displaying the same pattern of behavior.

1) 300 Win Mag, 210 Berger VLDs @ MV 2910 fps. Lilja 25.5" barrel. The rifle shoots less than 0.5 moa - no problems - both with this load and 200 gr Nosler ABs.

In order to obtain a left-right zero at 1000 yds, I have to adjust the scope such that the bullets strike 4 3/4" left at 300 yds.

2) 7mm Rem Mag Tikka T3, 168 Berger VLDs @ MV 3050 fps. Tikka factory barrel. This rifle also shoots less than 0.5 moa.

In order to obtain a left-right zero at 1000 yds, I have to adjust the scope such that the point of impact at 300 yds is 7.5" left.
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Anyone have an explanation for this? My bullets are drifting left to right. Spin drift? Voodoo? I don't turn target turrets for vertical or horizontal corrections. I use mil-dot reticles for holdover. So it doesn't have anything to do with turret twisting. I've read a little about spin drift and the Correalis? effect but didn't expect either of those, even combined, to account for this. Anyone have any ideas that might explain this?

I shot both rifles yesterday in cold, dead still, conditions in order to verify what I've been observing the last three times out, and there are now no doubts about it. My bullets walk from left to right.

 

[britz]

Are you using a scope level and is it trued with your rifle. If it isn't that could accound for the inconsistancy.

 

[kiwi3006]

Have a read of this thread, talks about spin drift which would probably account for your shots hitting to the right.

http://www.longrangehunting.com/forums/f17/308-win-spindrift-33446/

It contains a link to another website that has some calculations for determining the amount of spindrift.

Hope it helps.

Stu.

 

[royinidaho]

I'm gonna watch this one closely.

I have no ideas.

That is a bunch of shift

 

[MagMan]

Are you by chance shooting off a bipod on soft ground. I've often thought about the torque on the right side leg of a bipod driving into the ground and causing bullets to drift right, but I have nothing conclusive.

 

[squirrelduster]

It can't be spindrift the error is too great. Spindrift would be right on at 300 and an inch or so at 1000.
Sounds like your scope is not level.
Draw a plumb line on a piece of large paper at 200 yards and put a 1/2" or so dot on the bottom.
Shoot at 200 yards and make sure your shot is at the dot.
Crank 3 revolutions of the up knob on your scope and shoot again. Shot should be close to the line higher on the paper.
Do another 3 revolutions and see if you are still at the line. My bet is no.
If the answer is no then the scope reticle is not vertical.
Check this out from 6mmbr
Scope level -
Look at Catshooters description. It is really long but worth the reading. He is a little abrasive but knows his stuff.
Steve

 

[Pdvdh]

Thanks for your assistance.

britz - no scope level mounted on the scope tubes.

kiwi3006 - I will read that article and see if it helps.

royinidaho - All I can think is maybe both scopes are mounted slightly counterclockwise in the scope rings. Then when I level up the crosshairs with the horizon, the barrel may actually be a little to the left of dead center beneath the crosshairs. This would tend to send the bullet down the bore with an ever so slight rightwards trajectory. I was shooting along a set of power lines so I had the power poles and cross-members to ensure the scope's reticles were very close to plumb with the world and forces of gravity.

MagMan - No bipod. I'm shooting off a forearm sandbag rest.

squirrelduster - This could be the problem. I can't think of anything else. I measured between the flats on the scope rail and the flat on the bottom of the scope body under the turrets on my IOR 3-18X with feeler gauges and it's mounted pretty darn close to square. I will read through it and then ponder this some more.

If the line of the bore wasn't exactly under a plumb line under the cross hairs when I leveled up the scope reticles, then I can see how at the same time the bore is pointed up, it would also be pointed slightly over to the left or right. Is that what you're thinking?

 

[squirrelduster]

The scope reticle is very rarely EXACTLY plumb with the flat on the bottom of the scope or the the turret caps.

It must be exact for serious long range. Once you get the scope plumb use your power pole technique to get your rifle level and put a bubble level on your scope.

There are several made that work really well. I bought mine for my 300 win mag from Shawn at Defensive Edge. Great guy with great service. Look at the cosine indicator if you shoot in the mountains and you can mount the level and the indicator on the same mount.

Spin drift only applies to really long range from what I know about it. It was used for long range artillery calculations. Comes into play when you are shooting miles instead of yards.

The scope level thread is very interesting I printed it and have read it several times.

Good luck and let us know how you do.

 

[AJ Peacock]

Phorwath,


OK, bullet moving left to right?
Maybe it's compensating for your recent right to left political leanings?


In all seriousness though, the guys have given you all the likely causes.


Good luck,
AJ

 

[LewisH]

Maybe the base or the mounting srew holes are misaligned right to left relative to the bore.

Sounds similar to the problem encountered in shooting a side-mounted rifle scope (like on a M1 sniper rifle), but to a much lesser degree.

 

[Pdvdh]

Phorwath,


OK, bullet moving left to right?
Maybe it's compensating for your recent right to left political leanings?


In all seriousness though, the guys have given you all the likely causes.


Good luck,
AJ

Ha!!! I appreciate a good sense of humor! Caught me with my pants down on this one.

 

[Pdvdh]

Well if spin drift or some other kind of voodoo doesn't explain this then I'll work to ensure my scopes are both mounted in proper alignment with the bore. They reticles look straight with the gun and world, but maybe they're just a tad catty wompus. I'll let you know if/when I get to the bottom of this puzzler. Thanks.

 

[Pdvdh]

OK guys,

kiwi3006 provided a link to a LRH thread, which contained a link to this thread at the Sniper's Hide. Thanks kiwi.

Spindrift Calculation?? - Sniper's Hide Forums

Now I don't advise anyone to read that whole thread unless you're really into this stuff. It's a very, very long thread and it took me about 2-3 hours of time to digest the entire thread. Summarizing, the jist of it is that spindrift is for real, and with my rifles and bullets, I could expect to see about 10" (~1 moa) of left-to-right spindrift (based on right hand twist rifling) at 1000 yds. So spindrift could indeed explain most of the left-to-right drift I'm experiencing with my 300 Win Mag.

Spindrift doesn't account for the excessive left-to-right drift I'm experiencing with the 7mm Rem Mag. I'm getting about twice the drift attributed to spindrift with this setup. So I'm going to take a good close look at the scope set-up on my 7mm and make sure I haven't mounted the scope with a slight counter-clockwise rotation from vertical. If the scope were canted counterclockwise, then when I leveled the crosshairs, the bore would send the bullet downrange with a slight right drift. That's currently my best explanation.

Any other thoughts, I'm all ears. Other than that, I'll let you know how this pans out.

 

[Jim5351]

I don't know what bases you are using, solid or windage adjustable, but I had the same problem with my 300Win mag Leupold 4.5 x14LR with Leupold windage adjustable bases. If your scope is not in perfect alignment with your barrell for windage you will experience crossover. A lot of shooters, and shops for that matter, will mount a scope on windage adjustable bases, then adjust the windage using only the crosshair windage adjustment. It is very important to adjust the windage first by using the windage adjustment on the base, fine tune with the crosshair adjustment. It is very important to start with the crosshairs centered for windage. A very easy way to check this is to place a mirror against the objective of your scope, if you see two sets of windage crosshairs, the crosshairs are not centered. Bring the crosshairs back to zero and adjust windage first by adjusting the base adjustment. Hope this helps, it solved my problems