Uphill/Downhill ballistics (experiences?)...

[LongDuck]

This story was related to me by my Father-In-Law and I was trying to figure out exactly where the bullets were landing for the following shots:

1) 30.06 Browning automatic, 75-100 yard shot with 165gr boattail bullet. Large buck downhill from shooter at about a 45 degree down angle.

Where will the shot hit? (low to scope POA?)

2) after above shots miss, buck runs up to the top of the next ridge, approximately 45 degree *uphill* angle from shooter.

Where will this shot impact? (higher than scope zero?)

After trying to figure out where the physics affect the flight path, we're stumped. Any insight appreciated,

 

[Mike Irwin]

both uphill and downhill shots will hit lower than point of aim.

Don't ask me to explain it, I just remember it from Hunter Safety many years ago.

 

[bruels]

Shooting uphill or downhill both result in points of impact higher than normal. The correction for either is to aim low.

At 45 degrees up or down, a Federal .308 Match 168 grain BTHP will hit .7" higher at 100 yards, 3.3" higher at 200 yards, 7.8" higher at 300 yards, and 15" higher at 400 yards. Source: "The Ultimate Sniper" by Major John Plaster, USAR-Ret.

The reason for this phenomenon is the effect of gravity on the bullet's flight path is reduced as you approach vertical in either direction. If you were to shoot straight up or down in the air, gravity would only work to slow or accelerate the bullet. It would have no effect on changing the bullet's flight path from it's initial direction as it does when you fire horizontally.

It would be interesting to actually measure the ranges involved in your father-in-law's experience, find out what corrections he used, and compare them to tables to see just why he missed.

Another formula to use in computing correction, and it applies to any bullet's ballistics is at 45 degrees multiply bullet drop in inches times .293 and that is the correction in "hold-under" you must use. The formula also comes from "The Ultimate Sniper."




[This message has been edited by bruels (edited May 08, 2000).]

 

[Jeff, CA]

To follow up what bruels said: the horizontal distance (not line-of-sight distance) to the target is what determines the point of impact. At 45 degrees (up or down), the horizontal distance is 70.7% of the straight-line distance (71 yards, assuming 100 yard straightline distance in your situation). Draw a right triangle with one leg horizontal, one leg vertical, and the hypotenuse at 45 degrees. You're at one of the 45-degree corners, and the deer is at the other. The hypotenuse represents the line of sight between you and the deer.

Practically speaking, in your situation, aim as usual. The difference in POI is .7" - smaller than the diameter of a penny.