Help Sighting in at 100 yards down hill

Joined
Jul 22, 2009
Messages
1,192
Likes
189
Location
Hillsboro County, NH
Feedback: 16 / 0 / 0
I need some help / knowledge on sighting in a rifle at 100 yards, shooting downhill.

I have an M1A in .308 Win. I have a 100 yard range setup, but the target is downhill from my shooting position. The elevation change is probably 15-20'

What should I do about bullet drop and compensating for it? Would it still be the standard drop for the load used if shooting level?
 
At 20' in elevation, the horizontal distance would be 98 yards. So to sum it up for you, the height doesn't really matter at 100 yards, at 300 it might.
 
For such a slight incline, you can ignore the effect.

At a rise of 15' over 100 yards, the angle is 2.870 degrees and the cosine of that angle is .9987.

At a rise of 20' over 100 yards, the angle is 3.823 degrees and the cosine of that angle is .9978.

This means that for a rifle sighted for a theoretically perfect 100 yard zero on the level, for a shot on the incline the bullet will strike high by the same amount as if you shortened the horizontal distance to between 99.78 yards and 99.87 yards (i.e., as if you moved the target closer by about 7 inches). This is way smaller than your aiming error, so forget about it.
 
Here's the deal. The distance to the target is not the distance down the hill. It is the vertical distance. So imagine the target coming straight up in the air level to your line if sight. That is the real distance. Think triangle.

Here is a photo I found to help.

In the first picture the distance up the hill is 500 yards, but the horizontal distance is 433 yards. 433 yards is the actual distance. It doesn't matter if it is up hill or down hill. The distance is the same. In the second picture it is a steeper hill, but the horizontal distance is 250 yards, which is the real distance to the target.

uphill_shooting_angles.jpg
 
Last edited:
Back
Top Bottom