Scope Turret Calibration

[D.Camilleri]

I finally got out to shoot long range last night with my new barrel on my 338 rum. I was doing well at 690 yards, and since no one else was at the range, I went to the far birm and shot 986 yards. I had good paper on a 4x8 piece of plywood and thought I had good dope. I ranged the target with my G7 range finder and my balistic solution was 23 moa. My load is 300 gr berger otm @2800 fps. I took two shots and went to check the target. No holes! I looked very carefully and finally found one fresh hole in the plywood at the extreme top of the board above my paper. The impact was about 20 inches high. I went back and dialed 22 moa and took two more shots. 4 inches apart but still about 10 inches high. This doesn't match any of my drop charts!

This morning I took my rifle back out to the range and set the gun in a rest. I placed a tape measure on the plywood target at 100 yards. I aligned my cross hairs with the top of the plywood and zeroed the scope. I dialed 4 complete revolutions with my Sightron SIII and each 15 moa revolution moved right at 16 inches on my tape measure. 4 revolutions moved 48 inches.

So my question is how do I compensate for this variance? Using the G7, I can only compensate by changing the value for velocity. By changing from 2800 fps to 2900, it gets me close.

In my shooter app on my phone, I made a turret adjustment from .25 to .265 per click and it is also close. I now need to shoot to verify, maybe tonight when everyone else leaves the range.

Am I on track or should I do something different?

 

[ShtrRdy]

Some thoughts:

Did you have accurate numbers entered for temperature and station pressure?

You might also look at how much error in distance would be required to cause that much vertical difference? Maybe the rangefinder is a little off?

Are you shooting from a different position from what you used to zero the rifle/ammo?

 

[D.Camilleri]

A distance error would have to be very large. I am shooting prone compared to shooting from a bench. I am using the same front and rear rest. While shooting prone, I was laying in pea gravel. If there was a bigger difference in grouping, I would look at the pea gravel as a possible source, but with the shots coming in at 4 inches apart and taking one minute out of the elevation had a good change.

The G7 calculates temp and station pressure as well as elevation. I also ran the numbers on my shooter app on my phone and it was very close to the G7 and it produces station pressure and elevation also.

I just got this Sightron SIII back from the custom shop where I had them install a moa reticle and a tactical turret. With my old turret, I didn't have this issue.

 

[D.Camilleri]

I watched Brian Litz's video on doing a tall ladder test for scope tracking verification. I went to the range and set up 4 feet of vertical paper and drew a line with a sharpie while using a 4 foot level. I put a target dot at the bottom of the line. I center punched the target dot and dialed 30 moa up on my scope. Next shot, text book right on the line. Center to center of the shots was 31 7/8, goal was 31.70, scope is good!

I brought a shooting bench with me that I borrowed from a friend and set it up at 985 yards. My G7 called for 23 moa, I dialed and held about 5 inches right for wind and let two shots fly. One shot was directly in line with the bull and about 5 inches left (needed a little more for wind) and the other shot was also about 5 inches left and about 8 inches high. Someone else showed up at the range, so I couldn't shoot from the far location so I moved up to the main shooting line and took two shots at 690 yards and put both of them in the black. I used the dope from the G7 again without any variance. I guess the problem disappeared!

 

[Mikecr]

The problem hasn't gone away,, it will show up again.
31 7/8" = 31.875"
31.70" = ?
30 moa = 31.416" per hundred yards

Your scope is not dialing in actual MOA (regardless of it's merchandising). Based on earlier scenario it appears to me your scope is dialing slightly higher than MOA at 1.067 IPHY (per 4 clicks).
This is very common, and toy makers of every sort usually generalize right past it..
The problem is still that you need to be able to enter actual scope click value into basis for your ballistic solution.
Looking over the G7 brochure & manual, it doesn't appear that they gave you an option to do this.

Long range shooters need to think in terms of inches per hundred yards(IPHY). This is by far the highest resolution, allowing for no generalizations about it.
When we think in MOA & worse, MRAD, injected errors pile up pretty quick. Toy makers love for you to think in loose terms like these, as it separates them from accountability with inaccuracies.
So the clicks in you Sightron are not actually 1/4moa, and they're confident you'll never figure that out. Ballistic software makers quote scope brochures, to bypass specific input/ouput adjustments in solution. Now we have combined toys piling up the generalizations into a real mess..

 

[billigoat]

Mikecr,
Interesting post. I've always wondered if that was the case with some scopes. Thanks for the clarity.

 

[Mikecr]

From what I've read & with some testing on my part, I believe relatively few scopes actually adjust in true MOA. Of mine, NF NXS do not, my Leupold Mk4s do.
That is, Mk4s truly adjust at .262 IPHY per click.

Now you might generalize -what difference could these little numbers make?
If you take the notion that 1moa is ~1 inch per hundred yards, that's only 0.0118" off per click, right? How could that matter?
Well, multiply that error x 200 clicks(50moa) to get to 1kyds, and it adds to 2.36" off.
2.36@1kyd = .225moa
Where your accuracy goal is 1/2moa, losing 45% of it(.225/.500) by 1kyd -to error -sucks.
1/2moa of ACCURACY is tough enough for me without this error. I already have wind hold-off, Bullet variance, air densities, spin drift, horiz/vert coriolis, slope, ES, mirage, level,, and a horsefly on my nose..

I don't think OP's observation was purely due to scope, or that there was a scope 'problem'. But I believe some of the cure is a ballistic solution that accounts for actual IPHY of click value.

 

[D.Camilleri]

Mikecr,

I should have been a little more clear. My range from the bench to the target while doing the ladder test was 101 yards. According to the formula that Brian Litz uses, that gave me 31.70 with desired being 31 7/8. Let me know if I am missing something.