How does shimming a scope affect bullet trajectory?

[subscriber]

The OP did not mention the range at which he is trying to zero. If 25 yards, then a tall mount explains why he is having trouble.

OP, when you say 3" tall mount, is that measured from the center of the objective lens, or the top of the optic? The center of the optic element is the measure that matters.

[Starslinger]

I appreciate the responses everyone. Here are some answers to questions. The rifle is a Lee Armory Hunter with an RS Regulate side mount and an unknown brand of scope rings (they cost about $40 I think). Scope is Leupold VX-3HD 4.5-14x40 CDS-ZL. I have had problems with the scope, possibly due to the zero stop. The zero stop cap was temporarily removed and the scope has been adjusted down to the lowest possible elevation by a gunsmith. My preferred zero distance is 35 yards because that gives a reasonably flat trajectory out to 200 yards.

Once again, I am aware that an AK is not a good target rifle. I am aware that a red dot is more appropriate for an AK. I already have two AKs set up with red dots, but I want to push the distance a little further with my third and final AK. I do use cheap steel cased ammo and I am aware that this is also not ideal. However, I am only trying to reach 300 yards maximum and hit a 10x12 inch steel target. I can't do it with a red dot because I can't really see the target clearly at that distance.

I might try putting the scope on a different rifle using a different mounting system (A Saiga with a Texas Weapon Systems Dog Leg Rail). I own only AK rifles and .22 rifles. I do have a laser bore sight and I will try using it again.

I do not own a lead sled and my mechanical skills are minimal, but if I get desperate enough I may try Audiophil2's checklist. Here is a link to pictures:

https://imgur.com/a/f7vBIHT

Thanks again everyone!

[audiophil2]

Something is way off.
Not sure if the rail on the gun is crooked or the mount is crooked. The scope mounts look the same but maybe measure them to make sure they are the same height.
Is the scope contacting the blade?

[audiophil2]

I'm also wondering if you can't get a good eye relief on that. With a second focal plane scope eye placement is more important. If you are treating it like a red dot you will have problems.

[audiophil2]

Run to home Depot or Walmart and get 2 small bubble levels. One on barrel and one on elevation turret. Amazon has really small ones too. Level the barrel and see what the turret bubke.looks like.

[hermosabeach]

First off

Thanks for posting the photos


I would start simple

Place the rifle in a Vice - don't crush it- just slap a level on the barrel and clamp it down with the barrel level


Then set the level on top of the top turret cap

How far off is it?

You can remove the optic, set level in the lower half of the rings?

Is the bottom of the rings parallel to the barrel?

Keep working your way down and write down your findings.

Are the rings both the same height?

Place the level on the mount. Is it level with the barrel (parallel)

Try to work out the issue.

[five.five-six]

The scope has no effect on bullet trajectory. It's physically independent.

Exactly, the scope predicts the bullets trajectory.

[ar15barrels]

The rifle is a Lee Armory Hunter with an RS Regulate side mount and an unknown brand of scope rings (they cost about $40 I think).
My preferred zero distance is 35 yards because that gives a reasonably flat trajectory out to 200 yards.

I think your problem is related to trying to have a 35yd near zero with your 3" scope centerline height.
What you should do if you want to shoot out to 200 or 300yds is to set your far zero for 200 or 300yds and then just let your near zero fall where it may.
Your 35yd zero simply does not work for your cartridge and barrel length.

Here is a table showing the trajectories at various different zero distances.
Note how it's NOT possible to get your 35yd zero AND a 200 or 300yd zero at the same time.
That's simply the reality with your bullet's trajectory and your sight height setup.

 Tabular trajectory data at Std.ICAO Atmosphere
-------------------------------------------------------------------------------------
 Gun / Ammunition : 7.62 x 39 (.308) Russ.
 Bullet Type      : .310, 123, Hornady FMJ 3147
 Bullet Weight    : 123 grains or 7.97 Grams
 Muzzle Velocity  : 2350 fps
 Twist Length     : 11.0 in.  (RH)
 Gyro.  Stability : 3.82
 Crosswind Speed  : 10.0 Mph 
 Correction Factor of Elevation (MOA/Click): 1.375
 Ballistic Coefficient(s) (G1) :
 C1_1=0.266@V>0 fps;


 Table of Various Zero Ranges - New Zero-in Correction Values in Sight Adjustment Clicks 
-------------------------------------------------------------------------------------------------------------
 Range               25 yd.   50 yd.   75 yd.  100 yd.  125 yd.  150 yd.  175 yd.  200 yd.  225 yd.  250 yd. 
-------------------------------------------------------------------------------------------------------------
  25 yd. Zero           X       -3.6     -4.3     -4.4     -4.2     -3.8     -3.2     -2.6     -1.8     -1.0   
  50 yd. Zero          +3.6      X       -0.8     -0.8     -0.6     -0.2     +0.4     +1.0     +1.7     +2.5   
  75 yd. Zero          +4.3     +0.8      X       -0.1     +0.2     +0.6     +1.1     +1.8     +2.5     +3.3   
 100 yd. Zero          +4.4     +0.8     +0.1      X       +0.2     +0.7     +1.2     +1.9     +2.6     +3.4   
 125 yd. Zero          +4.2     +0.6     -0.2     -0.2      X       +0.4     +1.0     +1.6     +2.3     +3.1   
 150 yd. Zero          +3.8     +0.2     -0.6     -0.7     -0.4      X       +0.5     +1.2     +1.9     +2.7   
 175 yd. Zero          +3.2     -0.4     -1.1     -1.2     -1.0     -0.5      X       +0.6     +1.4     +2.2   
 200 yd. Zero          +2.6     -1.0     -1.8     -1.9     -1.6     -1.2     -0.6      X       +0.7     +1.5   
 225 yd. Zero          +1.8     -1.7     -2.5     -2.6     -2.3     -1.9     -1.4     -0.7      X       +0.8   
 250 yd. Zero          +1.0     -2.5     -3.3     -3.4     -3.1     -2.7     -2.2     -1.5     -0.8      X     
-------------------------------------------------------------------------------------------------------------
 181 yd. PB Zero       +3.1     -0.5     -1.3     -1.3     -1.1     -0.7     -0.1     +0.5     +1.2     +2.0   
-------------------------------------------------------------------------------------------------------------
Velocity       fps.  2272.1   2195.6   2120.6   2046.8   1974.6   1903.4   1834.5   1767.7   1702.8   1639.7   
Energy      ft.lbs.  1409.8   1316.5   1228.0   1144.1   1064.8    989.3    919.0    853.3    791.8    734.3   
Deflection      in.    -0.1     -0.4     -0.9     -1.5     -2.3     -3.3     -4.6     -6.2     -8.1    -10.3   
Correction  MOA/mph  -0.040   -0.076   -0.110   -0.142   -0.172   -0.208   -0.251   -0.297   -0.345   -0.392   
Time            sec   0.033    0.066    0.101    0.137    0.174    0.212    0.252    0.295    0.338    0.384

[kenl]

Think ARBarrles is onto something. Do you have a ballistic app that you can use to predict the approximate POI for your setup? If not you may want to get one.

I use the free android version of Strelok, but others will work. With it you can change a lot of things, like scope height, zero, etc. Use the cartridge info from ARBarrle's post and a starting BC of .275 (wolf classic military ball).

Looking at your photo, is the scope touching the front sight? That would cause issues. You might need some taller rings to clear that sight. And 3" isn't really that high for a scope mount. ARs with a scope in a cantilever mount are about 2.75" +/-.

Again, good luck

[foothillman]

Starslinger
The rifle is a Lee Armory Hunter with an RS Regulate side mount and an unknown brand of scope rings (they cost about $40 I think).
My preferred zero distance is 35 yards because that gives a reasonably flat trajectory out to 200 yards.

ar15barrels
I think your problem is related to trying to have a 35yd near zero with your 3" scope centerline height.
What you should do if you want to shoot out to 200 or 300yds is to set your far zero for 200 or 300yds and then just let your near zero fall where it may.
Your 35yd zero simply does not work for your cartridge and barrel length.

Here is a table showing the trajectories at various different zero distances.
Note how it's NOT possible to get your 35yd zero AND a 200 or 300yd zero at the same time.
That's simply the reality with your bullet's trajectory and your sight height setup.

I use this one, has both numeric and graph.
Advanced Options for zero and scope height.
If you can Zero @ 35yds than your scope is good for 300yds with 11" drop.

http://gundata.org/ballistic-calculator/

Look at the graph for your set up.
3" @ 35yds

3.5" high @130yds and 3.5" low @ 250yds
Large drop once you hit 300yds.
Also Zero Wind



300yds will be 11" lower, Silhouette Target 18"x30" plate should still ring @ 300yds.

[pacrat]

I think your problem is related to trying to have a 35yd near zero with your 3" scope centerline height.
What you should do if you want to shoot out to 200 or 300yds is to set your far zero for 200 or 300yds and then just let your near zero fall where it may.
Your 35yd zero simply does not work for your cartridge and barrel length.

Here is a table showing the trajectories at various different zero distances.
Note how it's NOT possible to get your 35yd zero AND a 200 or 300yd zero at the same time.
That's simply the reality with your bullet's trajectory and your sight height setup.

Same issue I addressed back in Post 16. I don't think OP is paying attention to answers he doesn't care to hear.

[tabascoz28]

It doesn't, it affects your perception of bullet trajectory. Gun powder, trigger control, barrel cleanliness, brass, bullet consistency/concentricity, humidity, air pressure really affect it.

[Starslinger]

Quote:
Originally Posted by ar15barrels View Post
I think your problem is related to trying to have a 35yd near zero with your 3" scope centerline height.
What you should do if you want to shoot out to 200 or 300yds is to set your far zero for 200 or 300yds and then just let your near zero fall where it may.
Your 35yd zero simply does not work for your cartridge and barrel length.

Here is a table showing the trajectories at various different zero distances.
Note how it's NOT possible to get your 35yd zero AND a 200 or 300yd zero at the same time.
That's simply the reality with your bullet's trajectory and your sight height setup.

Same issue I addressed back in Post 16. I don't think OP is paying attention to answers he doesn't care to hear.

OP here: I appreciate your input but I question why this would be true. I have red dots mounted at 2.75 to 3 inches height over bore and they zero to 35 yards just fine. Why would the scope be so different?

[pacrat]

Quote:
Originally Posted by ar15barrels View Post
I think your problem is related to trying to have a 35yd near zero with your 3" scope centerline height.
What you should do if you want to shoot out to 200 or 300yds is to set your far zero for 200 or 300yds and then just let your near zero fall where it may.
Your 35yd zero simply does not work for your cartridge and barrel length.

Here is a table showing the trajectories at various different zero distances.
Note how it's NOT possible to get your 35yd zero AND a 200 or 300yd zero at the same time.
That's simply the reality with your bullet's trajectory and your sight height setup.

Same issue I addressed back in Post 16. I don't think OP is paying attention to answers he doesn't care to hear.

OP here: I appreciate your input but I question why this would be true. I have red dots mounted at 2.75 to 3 inches height over bore and they zero to 35 yards just fine. Why would the scope be so different?

Because "red dots" aren't "scopes". They are different. A whole lot different when comparing a red dot to a 14x scope.

https://www.burrisoptics.com/blog/si...your%20reticle.

A red dot has a spherical mirror that reflects the light emitted from an LED of its axis focus. That spherical reflector has a special coating that only reflects red light. This prevents other light from disrupting your reticle.

To break it down simply an LED shines a beam of light onto a specially coated, slightly angled piece of glass. That glass reflects the light from an LED onto it, creating your reticle.

A scope is basically a tube with magnifying lenses at each end. With another tube inside it, that holds the crosshairs.

[foothillman]

I have a scope that sits 3 inches above the bore of the rifle. There is not enough downward elevation on the scope to zero the scope - it is shooting several inches too high. I have never done this but in theory if I shim or use adjustable elevation scope mounts so that the front of the scope is higher than the rear, I will be able to lower the point of aim/impact for the bullet and zero the scope, correct? So, how would that affect the trajectory of the bullet? How do I determine the bullet drop at various ranges for target shooting? Can anyone link me to a web based trajectory calculator that allows me to input the amount of shim and then calculates the trajectory? Thanks for any help!

Found this, hope it helps.

https://www.huntingpa.com/threads/how-to-shim-scope-mounts-on-rifle.25034/

Ok, this is really easy, but difficult to put into words.

slowly move your scope to a centered position, move it up down and count the clicks between all up and all down. Move to 1/2 of that distance/clicks. then it is elevation centered.
Then do a centering on the windage; all right --all left and count those clicks; then put it at 1/2 that number of clicks.

The internal adjustments on your scope are now centered.

Take it to the range and shoot at a large 3'x3' target at say 50 yards. shoot 5 shots and see how high or low the group is. For this say the group is 15 inches high at 50 yards.
Now it is a simple ratio of the distance and the amount high (or low) all in inches. in the example the 50 yards is 1800 inches so the ratio on a high group is 15"/1800" or .00835.
We apply that ratio to the distance between the front and the rear scope bases, assume that distance is 6 inches.

6" time 15/1800= .05inches If the group is high you will need to raise the front mount UP .05 inches. If the group is low then you need raise the REAR mount that amount.

Just buy some shims (Burris #625051) and place a .05 thick shim under the front mount (between the bottom of the mount and the receiver ring, tighten base screws and you are there.

I would follow his advice and start over.
OR
Swap scope mount positions front/rear

Using this with that
"several inches high @ 35 yards with adjustments centered"

2"/1260" = 0.0015873
Scope rings look about 4" or 5" ?

Soooooo shim the front
0.006" @ 4"
0.008' @ 5"
0.010" @ 6"
aluminum can about 0.004"

Did not research just using given information found on the WEB