Dope inconsistancy

[Canadian Bushman]

My rifle and data from shooter match up perfectly, except around 500yds. Ive tested this many times and the results are always similar. At about 450 my bullet starts hitting high, peaks at about .75-1.25moa above data around 500-525yds, then is spot on from 600+.

Ive gotten by with just using a dope chart built around average air density in my area, but id like to understand why this is happening. What am i missing guys?

 

[Michael Eichele]

I'd like to know as well. I had a load a few years ago that did the same thing only in reverse. It was low instead of high. Then at 700 it was where it was suppose to be.

This is only one of two things that has ever stumped me in LR shooting. I was using a scope that has always proven to have accurate tracking from 100-1000 yards with every other load and on several rifles.

 

[Canadian Bushman]

I never put the scope on another rifle but ive put different scopes on this same rifle and the "bump" is still there. I considered maybe a B.C., or a scope height problem but tweaking these things still doesnt make up the difference.

 

[bigngreen]

You need to take it farther out and get a broader sample.

 

[Canadian Bushman]

This particular round is a 308 berger 185gn that goes subsonic at 1250yds then gets pretty wild.

 

[Greyfox]

Are your 100 and 200 yard dopes exactly on?

 

[bigngreen]

Is there any terrain on the range, and have you checked to make sure that 500 yards is indeed 500 yards.

 

[Canadian Bushman]

Are your 100 and 200 yard dopes exactly on?

Yes. Many times i have moved forward to check 300, 200, then re zero at 100, then go back to 500 and shoot again. Even if i fudge the closer groups a .25-.5 moa low, there still ends up being about .5-.75 moa at 500yds of difference i can not account for.

 

[Canadian Bushman]

Is there any terrain on the range, and have you checked to make sure that 500 yards is indeed 500 yards.

I use a leica 1600 to range find, which it does very consistently. Ive pondered the idea of being off but the deviation in distance would have to be large (30-40yds) to account for the difference in drop.

Terrain is what i blamed this on for a good while. At my spot im shooting 500yds, im shooting over a small roll and then over a small bend in the creek. To maintain line of sight i always end up raising my bipod up to the point of instability. I always blamed the high hits on the way the rifle rolls during recoil and maybe the wind climbing the creek bank.

Until yesterday when i shot directly over row crop, with the wind to my back, with cold air and high pressure and still hit about 4" high. I cleaned this rifles barrel tuesday, and re zeroed weds with 15 shots to foul the bore.

I am shooting this bullet quite slow (2580 fps), would i need to tweak B.C.?

 

[royinidaho]

OP, Couldja post a pic of your shooting spot looking from the shot spot to the target.

I have a shooting spot where your condition occurs at a certain range with several rifles. Some of them no mine. :roll eyes:

I've had to shoot there under wind conditions from nearly zero velocity to 30 MPH to figure things out.

Our wind here is an average of 7 MPH from the same direction except in the morning when it comes from the opposite direction.

By noon the wind begins to be uncertain regarding direction. A mix of north-south currents. By 2 pm the dope is constant in the other direction and the "bubble" in the drop curve raises it's ugly head @ the 750 rock. The humiliating thing is that I seem to never remember this until after the first 750 yard shot.

 

[bigngreen]

Are you using a G1 or G7 BC, are you on out to a 1000 yards?

 

[Canadian Bushman]

Ok here is two pics, one looking at the target and one looking from the target.
The one looking at the target has a barn in the background. The target is small you may have to zoom in. Target azimuth is aprx 30 degrees north, north east.
Typically in the morning wind will blow from the north west and will move to the south east in the evening.

 

[Greyfox]

Given you scope is OK, and there are no conditions like wind effecting the trajectory, I would expect the answer lies in the BC/velocity settings. The only problem is that it would take a fairly large adjustment to the BC to effect the amount that you are off at 500 yards. The issue is more likely an error in both BC and velocity. The Berger/Litz BC's are generally accurate and would unlikely require the amount of correction to bring your 500 yard impact down. Having had a similar issue a few years ago I found that I solved the problem by first recording the current dopes. Next, adjust your velocity in the ballistic program, in your case, higher so your impact at 500 yards is corrected. Your program will now show your 700-1000 yards dopes low. Now adjust your BC lower to try and replicate the values of your original dopes for the longer ranges. I may take some trial and error. IMO

 

[Canadian Bushman]

Here is looking at target