Windage issues. Need some help.

[AndyRoo]

Hey guys, hoping someone can help me out with a problem I've been having for a while. Been trying to diagnose the problem for about a year. My .308 seems to consistently shoot to the right at longer range targets (500-700 yards). Gun is shooting consistently 0.5 mils right in very calm shooting conditions. First thought it could be a cant issue but after buying a scope level and remounting the scope to make sure everything is completely level I am pretty sure that is not the problem. Is there anything else that could be causing this unexplained right windage. I am stumped. Any help or suggestions will be appreciated. Thanks

The rifle I am dealing with is a factory .308 remington with 20" barrel and 1-12 twist. Bullet is a hornady 178 BTHP @ 2600 fps.

 

[Immike]

I believe spin drift is what you are experiencing. This comes from the right hand twist in your barrel. Mike

 

[Laurie]

I'm not sure if spin drift would account for that amount of movement. Bryan Litz's PM Solver II program gives the drift as 2.25, 3.4 and 4,92 inches at 500/600/700 yards for your combination assuming a 1-10" rifling twist rate.

I'm not au-fait with Mils, but I think these spin-drift values are all much less than 0.5 Mil which is equivalent to 1.7188-MOA according to a converter I've just consulted. 1.72-MOA = 12.6 inches at 700 yards, more than double what the program calculates.

Other possibilities are:

your barrel isn't 100% straight, not uncommon, and the small error is compounded by longer distances.

there is an issue in your rifling handling / its initial recoil on firing that produces the feature.

conditions aren't as still as you think. It's really rare for air to be 100% static. Unless there is a nearly always prevalent movement in one direction on this site though, you wouldn't expect the variation to always be one-way.

finally, and probably the primary cause - your scope doesn't just move the point of impact vertically as you change your elevation setting, but also moves the windage marginally to the right. If you want to test this out, use a clean paper sheet on a target frame at 100 yards, attach a plumb line to the top centre of the frame, make a mark top and bottom and draw a line between the two points to get a 100% vertical line. Affix / draw an aiming mark at or near the bottom of the line and zero the rifle and load onto it. Then shoot groups at your come-ups for 500 and 700 yards and see if they stay on the line. Any deviance shows the scope adjustment is the issue. Obviously this should be done in calm conditions and / or using sensitive vane type wind flags as well as shooting in benchrest or other supported conditions to minimise scatter.

This exercise was (still is?) highly recommended to Service Rifle competitors as foresight assemblies are often marginally out of true giving an inbuilt windage deviance as the range increased.

 

[jhord]

What have you leveled on the scope? The reticle, or the turrets?

 

[AndyRoo]

Thanks guys for the quick responses. I was always under the impression that spin drift was not worth accounting for until one starts shooting at distances further than 1000 yards, but that could be part of the right windage.

Laurie, thanks for all the suggestions. Never heard of crooked barrels, but I guess that could be the problem. Anyway of confirming that that is the problem. As far as the rifle handling, I'll be the first to say that Im not the best shot in the world but I've had other shooters use my rifle and experience the same results. Also meant to mention in my original post the scope is a Leupold mark 4, and that I conducted the exact test you described when I first noticed the windage issues. Scope tracks like a champ.

Thanks everyone. Keep the suggestions coming I'm desperate.

 

[AndyRoo]

What have you leveled on the scope? The reticle, or the turrets?

First leveled up the rifle using a level set on the scope base. Then leveled the scope reticle using a plum line set out at about 100 yards.

 

[jhord]

OK. If you have done the tracking test, and it tracks a perfect vertical line at 100, then you are looking at something more like your bases are out of kilter. IE, your scope is not parallel to the barrel.

 

[AndyRoo]

OK. If you have done the tracking test, and it tracks a perfect vertical line at 100, then you are looking at something more like your bases are out of kilter. IE, your scope is not parallel to the barrel.

I have thought about this as a possibility but didn't know if this would cause the problem that I am experiencing. Currently I am using leupold dual dovetail mounting system. These rings are the ones that are inserted into the base and then rotated 90 degrees to lock them into place. I suppose they could be lined up so that the scope is not parallel with the bore. Most long range rifle rigs I've seen do not utilize these but use picatinny base and rings instead. Is there a reason for this or is this just what most prefer. Thought about trying the picatinny base and rings as my next step to see if this solves the problem. Thanks for the help.

 

[jhord]

It could be any number of things. The holes for the bases not drilled right in the receiver, the barrel not square to the receiver, etc. Basically if the scope and barrel are pointing in different directions, you can only zero for windage at one point in space. Draw it out on paper and you will see what I mean. It takes a lot of time, perfect conditions, and a good dose of anal retentiveness to do it, but you can shoot at various distances and "zero" your windage by playing with scope cant. Or an easier way is to just develop a dope chart.