Does anyone know the proper basis or equation for working out bullet drop when shooting down or up hill?

I am trying to make a humble gadget to allow for this, to use with a cheaper rangefinder. The gadget is based on a piece of protractor and a plumb-bob.

My son has given me a list of figures based on the idea that it is the horizontal distance to where the target is. However, he now wonders if that is right basis, since time of flight is still much the same as for a level shot over the same (longer) distance.

Thanks if you can help

- Paul

Have a squiz here

http://www.loadammo.com/Topics/April04.htm

quote:

Originally posted by sambarman338:
Does anyone know the proper basis or equation for working out bullet drop when shooting down or up hill?

I am trying to make a humble gadget to allow for this, to use with a cheaper rangefinder. The gadget is based on a piece of protractor and a plumb-bob.

My son has given me a list of figures based on the idea that it is the horizontal distance to where the target is. However, he now wonders if that is right basis, since time of flight is still much the same as for a level shot over the same (longer) distance.

Thanks if you can help

- Paul

Easier to make one up using the cosine. That way you simply multiply by the cosine to get the corrected range.
Shoot me a PM if you want more detail.

Cheers, Dave.

Thanks John and Dave,

My son made up the tables using cosine and, from John's graphic, it looks close enough. On really steep shots, it seems to me some extra provision might also be added to account for the height of scope above the barrel and the departure angle (as seen in bullets striking above aim at 100 yards).

By that I mean a target 300 yards away, but 80 degrees below horizontal from the shooter, would only be about 52 yards out on a horizontal basis, and seemingly need to be shot straight at. However, if the rifle were set up to be two inches high at 100 yards, that departure angle would continue virtually unaffected by gravity, meaning the shooter would need to aim five or six inches underneath.

Does that logic make sense?

Interesting topic.

I am a visual concepts guy who aced in geometry but hated algebra though I am pretty good with number.

My thumb rule is as follows

If you are shooting at 400 meters with a 30 degree up or down slope, aim like a 300 meter level ground shot.

If it is 500 meters & 60 degree slope, again aim like a 300 meter level ground shot.

The trick is to get the slope angle estimate right. I tend to over estimate the slope.

According to No.1 son's table, you're not too far off, Nakihunter.

400m with 30 deg slope comes out as 346m (he deleted the fractions, so it could be 347).

The 60 deg sum is easy - it's always 50%, so the 500m (or yards) shot becomes 250m or yards. Beyond 60 deg I'd start to aim a little lower if your rifle shoots high at 100m. I haven't tested the tools yet, so it may all be BS.

Here's a couple of new questions.

How long is a big bull tahr from chest to tail?

How wide would such a beast be if standing perfectly straight on, facing you?

I'm going to make some targets for long range, which might also help me estimating ranges in relation to the Nikon's BDC reticle. I can probably find how high they stand at the shoulder but this is less important than, say, how deep they are through the chest in winter, because their legs would be hidden much of the time or hard to see at a long distance.

Of course critters doen't come in standard sizes but if I shoot over a small one's back, God bless him - he may grow into a big wise one.

Cheers
- Paul

Paul, from the older Leupold Rangefinders, like the RX II actually calculates everything for you. If you range an object on an incline or decline, the range it gives you is the range based on shooting level. Although the target is say 280 yds away, the reading it gives you is the yardage you should aim at, and the rangefinder might give you a reading of say 240yds, even though the target is much further. All you do is aim at the given range, calculates everything for you!

Mate I have used and researched the Burris eliminator rifle scope and this does it all for you, including illuminating a dot on the vericle part or the reticle to show you precise hold over. First time I used it in the field I shot a elk in Colorado at 540 yards. These scopes are now available here in Australia through most gun shops.

Thanks Marius,
I'll make note of your comments if/when I get a rangefinder.

Duggaboybuff,
I had come across the Burris system while checking out the ranging ability of different bullets/ammo but didn't realise it was quite that clever at the time. I've already ordered a Nikon, however, so might have to go with that.

Has anyone found anything on the bull-tahr-dimensions question?

- Paul

Paul, From top of shoulder to brisket just in front of leg is about 28 inch's. Thickness through chest is around 20. Thats just going on a mounted specimen. Remember there is also hair length to contend with as well.

S338 dont worry about the scientifics of it all so much,just put it on him and drill him!

Theres alot of truth in that. As Gryph will testify, in the country you will be hunting, often, whether its Chamois or Tahr, there is no time to do anything but shoot. It dosn't hurt to be prepared for the other occaisins though too.

Thats a really good simple method, and covers you for 99% of situations. I decided to try the newfangled longrange adjustable reticle scope out this last year, with my drop chart and all, and have never missed s much in my life. It's to slow and you have to think alot more when your not used to it. With a simple 3 inch high at 100 sighting, any steep angled shot I just aim 3/4 of the way down the chest, or use Tentmans method for longer shots. Anything over 450 and you can usually get closer anyway.

There are other ways too, as Shanks points out, often time doesn't allow for range finding etc.

A procedure taught to me (by a ex vietnam sniper no less) has always stood me in good stead . . . .

Make a quick mental estimate of the range (first thought, no optics, is usually best) - if you estimate its under 300 hold dead on.

If you estimate its over 300 use the animals top line as your aimpoint.

Cheers - Foster

Thanks men.
That's big, Craig, as it needs to be if these critters can be as heavy as a red stag. Obviously pics I've seen in mags can be deceptive but I can use that 28-inch chest depth to extrapolate the body length.

Gryphon,
I might go with a minimal rise above aim to limit that incline problem, but use the BDC for long shots.

Tentman,
Homer (formerly known as Hommer on another forum) said if you can seen the legs they are probably within 300 yards. Any thoughts on that? I notice the back legs tend to have a pale strip that might make them hard to see agains dry tussocks.

Cheers
- Paul

quote:

Originally posted by gryphon1:
Have a squiz here

http://www.loadammo.com/Topics/April04.htm

I can only hope that bloke's math is better than his English skills: "...by the same amount weather shooting uphill or down hill."

quote:

Originally posted by shankspony:
Paul, From top of shoulder to brisket just in front of leg is about 28 inch's. Thickness through chest is around 20. Thats just going on a mounted specimen. Remember there is also hair length to contend with as well.

Sorry Paul, the measurments I gave you didn't look right when I thought about them. Had a close look at my little ruler and theres a few inchs missing from the start.
make it 20 inches in height and 15 through the chest.

The horizontal distance / cosine is the rough easy way to adjust and for shorter distances will work OK - <500yds.
However the ballistic programs do use a much more complicated process to provide a more accurate come up for the longer ranges.

Thanks Craig,
yes, those new figures look more like what I see in pics of guys with tahr and rifles. I'm blowing up a pic - won't have to go quite so far now - but may paint it in the end.

Aussie Powder Burner,
500 yards should be plenty for my aspirations. As was pointed out earlier, the 270WSM is starting to run out of puff for tahr even before that.

I take it you burn Aussie powders. Do you know if they make 4350 under another name or an exact equivalent of it here?

Cheers
- Paul

AR2209 I think

Thanks Craig,
I waited a while in case anyone else had any opinion.

I was looking for a powder that would push both heavy and light bullets in the new rifle and someone said H4350 would suffice. 2209 will probably do the same in any case.

Cheers
- Paul