Ballistics question

[Uglyoldpoorman]

How can I find the answer to this question,,Would the study of external ballistics give me the answer

Question
Does one have to adjust their sites base the height from which they are shooting?

This question has been plaguing me and hopefully I can explain it well enough so the reader can understand why I trying to figure out if physics play a role in adjusting your sites based on the height from which you are shooting.

Say you adjust the sites on your rifle, while sitting in bench-rested position, and hit your target precisely where you aimed and X distance away at a target say 6' high.

1) Now you are in a ground level position (either in the prone or standing in a fox hole) and shooting at the same target, do you have to adjust your sites any since the bullet leaving your rifle has to over come some gravity to hit your target which is say 6'.

2) Now you are on a 200' cliff shooting down at the same target at X distance away, your bullet is aided by gravity, by shooting down, and thus it should fall more quickly than you were when you were in your bench-rested position at the same distance (X) away.

I know about adjusting for wind, distance, and other factors, but I am not sure how you would adjust based on varying heights from which you are shooting.

Does anybody know if or how this factor comes into play when shooting?

 

[ColinthePilot]

My guess is the difference would be negligible and your sights probably aren't precise enough to adjust for such small height changes. I'm no physicist, but I wouldn't worry about it. maybe a sniper or hardcore competition shooter could say for sure.

 

[Vern Humphrey]

Short answer, no.

While shooting up-hill or down-hill does have an effect, at normal ranges and normal angles, the effect is minimal.

 

[hankdatank1362]

2) Now you are on a 200' cliff shooting down at the same target at X distance away, your bullet is aided by gravity, by shooting down, and thus it should fall more quickly than you were when you were in your bench-rested position at the same distance (X) away.

This is a common mistake. Some people believe that gravity will speed up a projectile when aiming downhill or slow a projectile down when aiming uphill. A bullet, however, even the large, heavy .50 BMG, does not have sufficient weight to cause an appreciable change in trajectory or time-of-flight when fired up or downhill.

The trick, when firing up OR downhill, is to be able to accurately estimate your distance ot the target. Then, you must accurately estimate the angle up or down you are shooting (that's why they make those nifty angle/cosine bubble indicators) and multiply the slant by approximately :

.97 for a 15 degree slope

.87 for a 30 degree slope

.71 for a 45 degree slope

EXAMPLE - You see a deer on a hilltop about 310 yards away. You figure the angle to the deer is about 45 degrees from where you are. Mulitply 310 yards by .71 to get 220. 220 yards is the distance you correct your sights/scope for. Always aim as if you are shooting the shorter distance.

Remember, uphill or downhill makes no difference, only the angle.

I personally recommend Understanding Firearm Ballistics by Robert Rinker, if you are seriously interested in improving your understanding of projectile behaviors.

 

[Big Daddy K]

Good/interesting post but I think you are making it way too hard. Just shoot and have fun

 

[Vern Humphrey]

Correct -- but how often does on get a 310 yard shot up or down a 45 degree slope?

The greatest cause of errors when shooting up or down hill is foreshortening. The hunter is unable to accurately visualize the animal's vitals when viewing it at an angle.

 

[hankdatank1362]

Correct -- but how often does on get a 310 yard shot up or down a 45 degree slope?

Haha. Never... at least not where I live.

 

[shdwfx]

http://en.wikipedia.org/wiki/Rifleman's_rule

There's a nice little derivation at the end if you are mathematically inclined.

Remember, gravity and it's affect on bullet drop are independent of the projectile's weight. The only reason a .45 drops more than a .22-250 is the later gets to the target faster - they both drop at the same rate, 9.8 m/s^2.