20 moa bases

griffin86 said:
I have always wondered about 20 MOA bases. Consider the following Bullet drops for a 308

Yards Drop in inches
100 0
200 -4.2
300 -15.1

With a 20 moa base, does it change they way you aim? Do you still have to hold 4.2 inches higher than you aim at 200 yards?
Click to expand...

If the sloped base allows you to have a 100 yard zero, the trajectory at longer ranges will still be adjusted to zero with the same come-ups as with a flat base. The trajectory of the load doesn't change, but if the closest range you can zero is 300 yards, then you will have to hold off for close range hits. What does change is that you might be able to zero the rifle for 100 yards with the elevation adjustment of the scope bottomed out, allowing you to make use of the full elevation adjustment your scope has to offer, rather than only part of it.
 
This has got me thinking. Thanx guys I never though of it like this with angled bases. As the scope is moved forwards it becomes closer to the bore this affects how high the RETICULE is above the bore (the important part of the equation). The reticule will need to be zeroed at say 100yards. Bringing it nearly to where it was before you moved the scope forwards. There is however the tinny bit the reticule had dropped which must be very very small on a 0.33* angle if you moved the reticule forwards say 1"


Neat O
 
Good information, Boomer. On my hunting 30.06 I'm sighted in at 200 yards and can adjust the scope to 600 yards. At 100 yards the POI difference on a Moose is insignificant. However on a highly accurate paper puncher it could be significant.
 
I'll try this again. Moving the rings on a straight surface (same plane)will make no difference at all. Moving the scope forwards or backwards on an inclined surface (plane) will change the distance above the line of bore, but it will change the height equally at both ends. Given a rail with .020 difference from end to end and 10 slots that would be .002" per slot at both ends. That would change the impact on the target .002" so basically nothing changes. Now, if you shim inside one ring or shim the base the difference is significant, approximately 1 inch for every .001" based on a ring spacing of 5 inches. Distortion of the rail will do the same thing, 1 inch per .001. You are confuseing differring points on one plane, with establishing a different angle and plane.
The example of a rear sight raising and lowering would work if only the rear ring was on a ramp and the front remained stationary, but that is not what happens.
I'll assume your kitchen table is level. Put some identical dishes on it and they will be the same height. Move them around and they will still be the same height. Shim two legs at one end and the angle changes. Shim all the legs and the angle is restored.
 
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Dogleg said:
I'll try this again. Moving the rings on a straight surface (same plane)will make no difference at all. Moving the scope forwards or backwards on an inclined surface (plane) will change the distance above the line of bore, but it will change the height equally at both ends. Given a rail with .020 difference from end to end and 10 slots that would be .002" per slot at both ends. That would change the impact on the target .002" so basically nothing changes. Now, if you shim inside one ring or shim the base the difference is significant, approximately 1 inch for every .001" based on a ring spacing of 5 inches. Distortion of the rail will do the same thing, 1 inch per .001. You are confuseing differring points on one plane, with establishing a different angle and plane.
The example of a rear sight raising and lowering would work if only the rear ring was on a ramp and the front remained stationary, but that is not what happens.
I'll assume your kitchen table is level. Put some identical dishes on it and they will be the same height. Move them around and they will still be the same height. Shim two legs at one end and the angle changes. Shim all the legs and the angle is restored.
Click to expand...
I know it seems complicated but re-red the post carefully. Boomer is only sliding the scope on the rail. The angle doesn't change. What is changing is the height of the scope. The scope sits on an inclined plane to the plane of the bore. Sliding the scope up the plane increases the distance of the scope to the bore. Did I get that right, Boomer?
 
Gun5tuff said:
I know it seems complicated but re-red the post carefully. Boomer is only sliding the scope on the rail. The angle doesn't change. What is changing is the height of the scope. The scope sits on an inclined plane to the plane of the bore. Sliding the scope up the plane increases the distance of the scope to the bore. Did I get that right, Boomer?
Click to expand...

The distance will change approximately .002 at both ends of the scope per notch and as the angle doesn't change the total change on target will be .002".
 
griffin86 said:
I have always wondered about 20 MOA bases. Consider the following Bullet drops for a 308

Yards Drop in inches
100 0
200 -4.2
300 -15.1

With a 20 moa base, does it change they way you aim? Do you still have to hold 4.2 inches higher than you aim at 200 yards?
Click to expand...

No, installing a 20moa base won't change the way you aim.

What will happen is this, you will find your scope aiming "down" 20MOA relative to where it would be pointing with a base without any slope to it. This means that at 100 yards, your crosshairs will be looking 20" lower than they otherwise would be. So if you then raise your rifle so that your crosshairs are pointing at the middle of the target, what you have done is made the barrel aim 20" higher. So if you were to fire a shot, it would strike 20" higher than you aimed it.

So then, in order to get your rifle to hit your your point of aim at 100y, you will need to turn your scope elevation adjustment DOWN by 20 MOA (hopefully you have that much "down" available!). In some scopes, you will be nearly at the scope's mechanical bottom range of motion. Then, this becomes your 100y zero. The good news is that if you want to shoot at longer distances, you now have 20MOA more "UP" elevation available to you before your scope runs out of elevations movement.


Crankbender said:
This has got me thinking. Thanx guys I never though of it like this with angled bases. As the scope is moved forwards it becomes closer to the bore this affects how high the RETICULE is above the bore (the important part of the equation). The reticule will need to be zeroed at say 100yards. Bringing it nearly to where it was before you moved the scope forwards. There is however the tinny bit the reticule had dropped which must be very very small on a 0.33* angle if you moved the reticule forwards say 1"


Neat O
Click to expand...

If you raise your scope an inch higher, this is exactly the same as lowering your bore by an inch. So your shots will land one inch lower, at all distances, as a result of this.

A degree is a slope of about 1 in 60, so a third of a degree (20MOA = 20/60 = 1/3 degree) is a slope of about 1 in 180. So, sliding a scope forward one inch, will lower the scope 1/180", which is about five thousandths of an inch. So your bullet will strike five thou higher, at all distances, than it otherwise would have (in other words, you will *NEVER* see this difference...!)
 
rnbra-shooter said:
A degree is a slope of about 1 in 60, so a third of a degree (20MOA = 20/60 = 1/3 degree) is a slope of about 1 in 180. So, sliding a scope forward one inch, will lower the scope 1/180", which is about five thousandths of an inch. So your bullet will strike five thou higher, at all distances, than it otherwise would have (in other words, you will *NEVER* see this difference...!)
Click to expand...


But there still is a mathematical difference that I had never contemplated before this thread. That part is the real value for me, even if it is truly meaningless in real life.
 
If you want to calculate the vertical change in the scope's position, the formula is Height=d x tan.3, where d is the distance along the base the scope is moved.
 
Looks like I have some back peddling to do. I just put the bore sighter on my rifle with the scope first in the forward position then again in the rearward position 2 notches back. I must confess the difference is nowhere near as much as I had recalled, but it is there and measurable, at 2 MOA. Now 2 MOA is far different than the 24 MOA I thought I had corrected, and so in practical terms Dogleg and Tiriaq are correct, moving the scope forward will not make a dramatic change to the elevation. In the case of my rifle and my base, for each notch the ring moves, the elevation changes 1 MOA.
 
Boomer, if you are talking about putting the scope at a different fore/aft position on a single-piece base, there should not be any observable difference (presumably you are talking about where the reticle is pointing, with an optical collimator mounted in the muzzle?). If you are seeing 2MOA, then something is wrong - perhaps the base is bent (not likely), perhaps the rings aren't properly connecting (square and stress-free) to the base, etc.
 
Simple explanation is it's thicker at the back than the front, making your scope point down. You put your rifle up and level it to look at the target your barrell is now pointed farther up making you shoot a higher arc or automatically making you aim higher.
 
rnbra-shooter said:
Boomer, if you are talking about putting the scope at a different fore/aft position on a single-piece base, there should not be any observable difference (presumably you are talking about where the reticle is pointing, with an optical collimator mounted in the muzzle?). If you are seeing 2MOA, then something is wrong - perhaps the base is bent (not likely), perhaps the rings aren't properly connecting (square and stress-free) to the base, etc.
Click to expand...

Those Badger Ordnance bases are pretty thick , so I don't think there is any bending going on. It could be that my particular M-700 has an out of spec receiver, but visually there is no apparent issue with the contact surfaces. The BO rings on my old scope makes good contact with the base, as does the one piece ring arrangement on the S&B. I checked both scopes against the bore sighter grid, and both show a 2 MOA change when they are moved two notches.
 
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