Interesting mathematical question

diopter

diopter

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Shot one of my commercial Yugoslav Kraguvac Mausers that I couldn't get the scope to zero today outdoors.
Ran out of adjustment to lower elevation. Still shoots 5 inches high at 25 YARDS!
Scope is a cheapy Bushnell 3-9x Banner on Weaver bases with Leopold weaver rings.

How much( in thousands of an inch) would I have to lower the rear base, or alternatively raise the front base?

Using Hornady's ballistic calculator, I get 25yd height of 5.1 inches at 25yds with a zero range of 625 yds!

Used data for a 140gr 7mm bullet , BC of 0.392 at 2450fps. with a measured scope height of 1.8 inches.
 
I won’t be the one to answer this question but I think we need to know the distance between the bases / rings to give you an accurate answer. Long action / short action lengths would effect the amount of shimming I believe.
 
Outside of scope bases 5.25 inches apart. Typical Yugo 0.250±inch shorter intermediate length bolt than a k98K action.
 
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High divided by distance = ratio (slope)

5" divided by 900" (36"x 25yds)= 0.005555556 target

.006(rounded up) x 5.25" scope ring distance =0.0315" Δ ring base height. (minimum)
 
If you are considering .030" or so shim under one base, you will want to verify ring concentricity with a lapping bar - if they are in line now, they will not be in line after installing a shim that thick. The rings won't "tip" to match the new angle - instead your scope tube will bend. You might consider the Burris rings with the off-set inserts - I think you would need a +.020" at one end and a -.020" at the other end to get your scope adjustments away from their maximum adjustments.
 
Check to make sure the bases are correct would be my first idea, depending where you got the bases they can get mixed up or in some instances the front and back bases reversed. As mentioned if the bases are correct then Burris offset rings would be the best choice.
 
b72471 said:
Start with a 1/4"=.250. I have raised open sites to shoot 1000mts and it was more yet, on model 96 Swede.
Click to expand...

Was wondering what you were talking about, until I took the rear base off and measured it's thickness. Mine is .0274" thick through the middle.
 
I ran into this issue on a No 4 with a older Weaver scope. The scope was the issue. The adjustments would not allow the scope to center vertically. When viewed with the objective on a mirror you should see 2 sets of crosshairs and be able to adjust the scope until they align, centering the scope. I would verify the scope first.
There is a formula in the Lyman reloading manual which allows you to calculate sight adjustments required which could be adapted to your purpose I believe.
 
Took off about 14 thou of the bottom of the rear base (Weaver #45) yesterday and added a 5 thou brass shim under the front base(weaver #46).
Found a matching diameter broom stick, tried various grit sandpapers and settled on P80 3M disc to wrap around the broomstick. relatively easy with that grit but still takes time.
Making the screw hole for shim took longer than expected.

Bottom of rear base as it was originally mounted, before I started sanding.
Yes, charger guide is still on receiver.
20191229-193359.jpg

Will try it out at 50m later this week.
 
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