14/1000"

[Mike Shoots Stuff]

In one of the episodes talking about rear bags (and in several other episodes), Frank talks about a 20 MOA sight base and how it only needs 14/1000" between the front and the back of the base to create a 20 MOA offset.
There is a formula to find how much you need to offset two points of a known distance to get a linear offset at a known range. I learned this while shooting High Power. It only takes 8/1000" between the front and rear sights of an M14 to change the POI by 1 MOA. That's the thickness of two human hairs. This is why sight alignment is so important. For the AR10 and AR15, this is about 6/1000" with a 21" sight radius.

Here is the formula.

(POI correction in inches) X (sight radius in inches) = Sight Change in inches
(distance to target in inches)

Using the M14 example...
1.047" X 26.75" = .00777979 (rounded to 8/1000"
3600"

If you replace the sight radius with the distance between the front and rear rest you can find out how many thousandths of an inch it will take to move one MOA.

I used this formula to make a "1000 yard" front sight for my AR10 by taking 0.091" of the front sight, with brings the POI up by 15 MOA. Doing this prevents me from raising my rear sight so much that I'm loosing cheek weld.

 

[C.R. Adams]

Nice work.

Another solution to solve is................ inch change @ target/inches to target = inches change @ front sight/sight radius in inches................................. two fractions equal to one another. The user will know three of the four, simply solve for the unknown one (which is what your formula is accomplishing if the unknown is sight change). These formulas are good enough for gov't work at reasonable distances. However, at extended distances, it gets a bit more complicated as the bullet doe not stay on straight line past a certain distance.

 

[Mike Shoots Stuff]

These formulas are good enough for gov't work at reasonable distances. However, at extended distances, it gets a bit more complicated as the bullet doe not stay on straight line past a certain distance.

Thanks. Although I need to give credit to Kenneth Royce, writer of Boston's Gun Bible. This formula pertains only to physically changing the sights to affect LOS. Are you referring to spin drift?

 

[C.R. Adams]

Thanks. Although I need to give credit to Kenneth Royce, writer of Boston's Gun Bible. This formula pertains only to physically changing the sights to affect LOS. Are you referring to spin drift?

spin drift would be one component. there are quite a few of external ballistics variables that would impact the outcome. Both, your and my examples/formulas work on the assumption the bullet travels in in a perfectly linear line from point a to point b. However, as you know, the bullet behaves more of an arc.

 

[Mike Shoots Stuff]

spin drift would be one component. there are quite a few of external ballistics variables that would impact the outcome. Both, your and my examples/formulas work on the assumption the bullet travels in in a perfectly linear line from point a to point b. However, as you know, the bullet behaves more of an arc.

Right. I'm talking just about movement of the rifle separate from external ballistics. This is about sight alignment and small movements of the rifle. I used this formula to drift iron sights for windage vs. using my windage knob.