Reticle Adjustment Question

[NewB7654]

If a scope had 60 moa vertical and 60 moa horizontal, does that mean from center, you can adjust left 60, right 60 and as well from center up 60 and down 60?



That's what I was told which led me to ponder - why in the world would anyone need to lower the bullet up to 60 MOA? All my bullets drop as soon as the leave the barrel, who needs to drop them further? & why?

 

[robert6715]

If your scope has 60 MOA of adjustment, then its 30 up, 30 down, 30 left, and 30 right if its centered to begin with.

Hope this helps, Rob

 

[NewB7654]

That didnt help answer my question at all. Even if its 30 all 4 ways, or 45 or 60, the question remains.
Since a bullet always drops, who and why would you need to drop the bullet even more by adjusting for it rising to much?

 

[LouBoyd]

That's why 20 MOA (and even greater) wedged scope bases are popular. So you can use more of the scope's adjustment range. Not all scopes have symmetrical adjustments though the majority do. Examples of scopes which don't are the Horus Falcon with the H37 reticle where
the reticle itself is offset 20 MOA and the Leuplld Mk 4 16X40 M1 where the target knob simply has more range of adjustment in the vertical axis even though the reticle appears symmetrical looking through the eyepiece.

You could also ask why scopes are made with only +/- 15 moa or less adjustment at all, , but that's'all that's need if you only shoot game or varmints or bemchrest competition at 200 yards or less.. Not everyone want's to shoot at 1000 yards. or even 200.

 

[bruce_ventura]

For two reasons that I can think of. First, to exploit the full adjustment range, a tilted base is sometimes used. 10, 20 or 30 MOA of elevation can be added this way. Second, crooked barrel installation, machining errors on the receiver or base, and barrel vibration can cause the shot to land high. In both cases the elevation has to be turned down from the center to sight in the scope at 100 yds.

 

[NewB7654]

A wedge or tilted base is intersting. But Ive always heard when mounting a scope, make sure it's perfectly level with the barrel.
If I use a 10 MOA tilt or wedge, sight in at 200 or so, will the adjustments at other distances still be the same -10 MOA?
If I use to raise the bullet 25 moa at 1000 will it now only be 15 with a wedge and its just that simple for all drop adjustments?

 

[LouBoyd]

A wedge or tilted base is intersting. But Ive always heard when mounting a scope, make sure it's perfectly level with the barrel.
If I use a 10 MOA tilt or wedge, sight in at 200 or so, will the adjustments at other distances still be the same -10 MOA?
If I use to raise the bullet 25 moa at 1000 will it now only be 15 with a wedge and its just that simple for all drop adjustments?[/QUOTE

The veritical axis on this drawing is greatly exaggerated but it gives an idea of what a ramped (slanted, tilted, whatever) scope base achieves:
http://img.photobucket.com/albums/v257/Erkorene/slantedscopebases.pngf

As long as the angle and separation of the optical axis and the bore remain the same when zeroed at the same distance all of the drops whether in mils or moa remain the same. The scope base does not change the trajectory of the bullet since the knobs on the scope are adjusted by the same angle to cancel out the added angle of the base. The aount of base wedge you sellect should a little less than half of the scopes adjustment range. Fore example, if you scope has 60 MOA of adjustment range (end to end) then a 20MOA base would be practical. I have 40 MOA wedges on a couple of very long range target rifles which have scopes with over 100 MOA of internal adjustment.

 

[NewB7654]

And another thought. All the ballistic calculators I use ask for your sight height. If the sight is tilted, what do you use for sight height?

 

[bruce_ventura]

Calculated bullet drop values are not very sensitive to small errors in sight height value. 20 MOA of base tilt amounts to a very small height change across the base (about 0.030"). An error in the sight height value of 0.030" changes the point of aim only 0.30" at 1,100 yds, assuming a 100 yd zero.

I typically use the height at the center of the objective lens because this lens positions the optical axis of the scope.

 

[LouBoyd]

And another thought. All the ballistic calculators I use ask for your sight height. If the sight is tilted, what do you use for sight height?

That depends on how the particular ballistics program was written, but most (based around the work done by Robert McCoy of BRL years ago called McTraj) use the muzzle for referfence zero point for all calcations. Assuming the target is horizontal to the shooter it's the vertical distance from the center of the line of sight to the center of the muzzle.
Unless the angle is radical the height of the center of the scope tube above the center line of the receiver is good enough.