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Sighting in rifle downhill?

Old 08-26-2007, 05:45 PM
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Nontypical Buck
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Default Sighting in rifle downhill?

The area where I sight in my rifle is in an old pit on my property. The 50 and 100 and 150 yard positions are all nice and flat and work well, but I'm wanting to work a little on my BDC drop rectile and move out a little farther. If I go out to 200 yards I will have to go up on the top of the pit and shoot down into it. There would be about a 15 foot elevation drop from the top of the pit down to the target. I still don't know if I'll be able to get out to 300 or 400 yards and still see the target so it might all be a moot point.

How much would the 15' elevation drop affect the impact point of the bullet at 200, 300 and 400 yards? A fraction of an inch or would it actually be a meaningful difference?

P.S. I'm shooting a 7mm Rem Mag.

Thanks, Nathan
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Old 08-26-2007, 06:48 PM
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Nontypical Buck
 
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Default RE: Sighting in rifle downhill?

How much would the 15' elevation drop affect the impact point of the bullet at 200, 300 and 400 yards? A fraction of an inch or would it actually be a meaningful difference?
The amount higher the impacts would be are insignificant at 200 yards and beyond.

A fraction of an inch is a lot closer than estimates of a foot.
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Old 08-26-2007, 06:52 PM
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Default RE: Sighting in rifle downhill?

Agreed. The angle isn't enough to make a noticable difference in impact.
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Old 08-26-2007, 08:56 PM
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Default RE: Sighting in rifle downhill?

I suggest you shot paper a200, 300, 400 from that higher vantage point and you can see where it shoots.

Try this . . . whatever your balistics are, zero that much high at 100 yards so you would be zero at 200. For instance, .300 win mag in 180 gr nosler zeroed at 200 is 1.5" high at 100. Then shoot from up the hill at 200 and see if it's still dead on or not. I think you will find that it is shooting a fraction high because you need to aim low whether shooting uphill or downhill. Capice? How much off it is will tell you if there is a drop differential.
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Old 08-27-2007, 09:50 PM
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Nontypical Buck
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Default RE: Sighting in rifle downhill?

Thanks for the input everyone. I'll give it a try soon. With the 7mm Rem Mag I could probably just sight it in dead on at 200 yards and ignore the BDC rectile,butI'm going to see how it works. Unless I can improve my shooting over the next month or so I'm not going to attempt any shots over 300 yards anyway.
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Old 08-27-2007, 10:15 PM
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Default RE: Sighting in rifle downhill?

I think what you're supposed to do with the BDC is zero at 100, then shoot 200 using the 200 crosshairs and just see where it's shooting. I doubt it would be right on the crosshairs even on flat land. Scope companies have a good idea with the BDC's, but just can't make them to exactly shoot for all. Even if you had one for 7mm, it would be off depending on the bullet, powder, cartridge, etc. you were shooting.
You juts have to move out, establish a pattern and remember where it shoots at 200, 300, and beyond.
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Old 08-27-2007, 10:31 PM
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Default RE: Sighting in rifle downhill?

I am looking in my Sniper Log Book / cheat sheet on this one.Depending on the situation, use this situation to get your correct shooting distance for Angle shooting. This is hard to describe without a picture as far as WHY. Think of it this way:

you are on a slope shooting downward you are ranging from your location the the target. that is a longer range then you are actually shooting. Draw a triangle on a piece of paper. straight on the bottom, straight up on a side and the line to the bottom. At the top, is you, and the bottom of the slope is your target. That sloped line is longer than the bottom line. The bottom is the actual distance that the bullet is traveling. That is why you tend to shot higher on downhill shots. The higher the angle, the longer the shot, the more deviation you are goiing to have. I was always taught that it the elevation is less than 2 stories (20-25 feet) then you don't have to worry, if it is more, then you need to do the math here.

SO here is the Formula. YOU WILL NEED TO KNOW WHAT ANGLE DOWN YOU ARE SHOOTING ON. (there is a great device out there that mounts on your scope to give you shot angle)

(G-T) = gun to target distance as per your rangefinder
COS = angle cosign

Angle / COS
10.98
20 .94
30 .85
40.75
50 .65
60 .50
70 .35
80.20

Formula:

(G-T) x cosign of angle = actual distance to dial in on scope.

I suggest taking an hour one day, and doing a cheat card for your rifles that has in 25 meter increments, the correct angle shooting info for your rifle. I kept mine, and my scope adjustment sheet for given distance taped to the stock of my rifle for wuick reference, and so i never was without it.

Hope this offers some help, and does not confuse you too much. Let me know if you have any questions about this.


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