POI change?

sniperman

hi all,was out with me hmr yesterday to check zero at 100 yrds,all was fine,groups within 1",so as i was out, decided to go to a sand quarry i have permission on, to have a few shots at rabbits,went up on a bit of a hill looking down,about 10 feet up, so im now shooting downward.first rabbit popped out about 90 yrds,bang,about 2" over head,same with the next 4,thought something wrong,went back down to level ground to check zero again,she was bang on,,so my question is would POI change form shooting level 1"groups to shooting 2" high downwards,any thoughts/advice welcome,

Cass

Shooting uphill or downhill can have an effect on POI as the bullet is effected in horizontal distance ranther than "line of sight". Bear with me on this as it's awkward to explain.

If you are shooting on the level at 100 yards and your gun is zeroed for 100 yards then you'll hit your target. If you are on a raised platform then the height this raises you can increase the "line of sight". So while you range it at 100 yards (still your zero distance) your bullet will only be effect over the horizontal distance which may only be 80 yards, depending on height raised. Here is a quick diagram. It's very basic, but i only have MS paint.




So in the diagram you see your shooting position is raised and while you see/range 100 yards to the target you are actually only going to have an effect on your bullet over 80 yards. As you know when you are closer to your target than your zero distance your shot will go high. So you aim low to compensate.

The funny part of this is the same thing is true when shooting uphill. So if the diagram above was reversed and the shooter was shooting uphill you still aim low as the same principles apply. This is obviously dependent on your zero, etc.

Now this is all well and good for a debate on ballistics, dynamics, etc. but for real world situations the 10 feet you speak of could not affect the bullet's performance to any real degree. So once you are within your 100 yard zero you should still be hitting spot on. I mean a rise of 10 feet might increase the distance from 100 to 101 yards. So nothing.

I would suggest you check your distances. Are you sure the distance from your shooting position to the "targets" is 100 yards? (give or take a couple of yards) It sounds to me as though you are closer than 100 and hence getting high strikes. If using a range finder be aware that some may not range correctly depending on the material hey are "bouncing off" to get the range. If guessing then i'd walk it to make sure.

I would hesitate to blame the rifle/rig as if it's hitting spot on when level then it cannot be anything there.

meathstevie

Cass, if you're shooting from a hill or not isn't it still the distance between muzzle and target that matters more so than the actual horizontal distance between shooting position and target ? At the end of it all the bullet still needs to travel from the gun to target and not from a theoritical point somewhere in the base of the hill ?

Cass

A bullet's trajectory is curved, and we get drop, ONLY because of gravity acting on the bullet. Gravity does not "allow" for uphill or downhill shots. It only acts upon the bullet over a given horizontal distance and not "line of sight" distance.

EDIT - I read back this post and it was not well explained so i've changed it. The line of sight is not as important as the horizontal distance traveled. A Mythbuster's episode reminded me of something. An example would be to shoot straight down/up. If you put a target on the ground and were raised 300 yards directly up from the target with a rifle (regardless of the rifles zero) and aimed at the target the bullet would strike the target because there is no horizontal effect from gravity on the bullet. Meaning there is no curvature of the bullet and it only stopped because the ground/target acted upon it to stop it. As the angle of the shot becomes more and more gravity has a "longer chance" to act on the bullet and so the amount of curvature (the trajectory) of the bullet becomes greater.

Hence the reason people use angle degree compensator if they take a lot of shots up or downhill. Also the reason you can buy rangefinders with this built in.

everypenny

Cass +1


Also note that if you are shooting uphill the same rule applies. Although it looks like you are shooting for 100 m in the diagram above the bullet is only being affected by gravity for 80 m.


Gravity is the main force in the drop of a round.

Cass

everypenny wrote: »

Also note that if you are shooting uphill the same rule applies. Although it looks like you are shooting for 100 m in the diagram above the bullet is only being affected by gravity for 80 m.

Yup. As i said above:

Cass wrote: »

The funny part of this is the same thing is true when shooting uphill. So if the diagram above was reversed and the shooter was shooting uphill you still aim low as the same principles apply. This is obviously dependent on your zero, etc.

I'm not going to pretend to understand the physics behind all things shooting. I'm no where near clever enough to. Newton's first law, and other factors i don't understand come into play. However as shooters we don't need to know it all. As a rule of thumb, depending on your zero/distance to target (HD), usually aim low. Here are a few examples (all shooting downhill as in the diagram).

With a rifle with a 100 yard zero (LSD = Line of Sight Distance & HD = Horizontal Distance):

• Distance to target is 120 yards (LSD). HD is 100 yards. Aim bang on.
• Distance to target is 100 yards (LSD). HD is 80 yards. Aim low.
• Distance to target is 190 yards (LSD). HD is 150 yards. Aim high.

These numbers are for illustration/example purposes only. To get true numbers just check online. There'll be a compensator/calculator/chart for it somewhere.

everypenny

Cass wrote: »

Yup. As i said above:

Sorry, missed that!


If you really want to go into it then cast your mind back to maths in school and all that trigonometry that you never thought you'd ever use.


Every angle has an equivalent cosine. If you don't believe me then here's NASA backing me up
http://www.grc.nasa.gov/WWW/K-12/airplane/tablcos.html


The COSINE is a great bit of gear. You can use it to figure out your horizontal distance to a target. All you do is multiply your Line of Sight range (say 100 m) by the COSINE of the angle you're shooting at.


Lets say its 45 degrees. As you can see in the table above the COSINE of 45 degrees is .7071, which we round up to .71
Multiply your range (100) by this and you get what you should actually be holding for that target/angle which is a range of 71 meters. Make sense?


Working out angles can be very tricky, and obviously you can't guesstimate, especially at longer distances where an error can turn in to meters off target!
But luckily there are a few products on the market. One that I like (and if anyone wants to buy me one for a present feel free to PM me) is an Angle Cosine Indicator.
http://accurateshooter.net/Blog/wardaci01.jpg


You can get one with just angle, just cosine or both. They're mechanical and so don't need batteries and you can attach them to a picatinny rail on your rifle.


Then all you need is a beautiful mind or a phone calculator to do the math.


Here's a good video on it.
https://www.youtube.com/watch?v=95bPADBjkbI

Cass

Ahhhhhhhhhhhhh. My head hurts.



Fair play for taking the time to write that. It can be complicated, but really we don't need to know too much about it. If people have an interest then by all means check it out, but sometimes over thinking something will result in a missed shot. Give every situation the necessary amount of thought and rely on your experience/training.

extremetaz

That flat line method is a very interesting way of explaining the calculation alright, and sufficiently accurate for the application - my inner maths geek is screaming but yeah, there's no sense in overcomplicating things and I can't see that there'd be any real world reward to it.

sniperman

thanks lads for all the good write ups,some great details,really helpful as always;)

everypenny

extremetaz wrote: »

That flat line method is a very interesting way of explaining the calculation alright, and sufficiently accurate for the application - my inner maths geek is screaming but yeah, there's no sense in overcomplicating things and I can't see that there'd be any real world reward to it.

There is a big reward if you're shooting out to a big distance or from a large angle though. Shooting at 100 meters from only a 25 degree slope changes the range to 90 meters. (COS of 25 degrees =.9)

At a 1000 meters you'd be aiming for 900 and on a .308 thats a difference of 7 feet from point of aim to point of impact.

It would want to be a tall rabbit

Cass

Nearly forgot the OP. Sorry lad.

It does relate to the posts though.

• Are you sure of the actual distance you are shooting at?
• Is the height of the ground only 10 feet?
• Have you a range finder or other measuring device?

If the height is only 10 feet then it's not enough of an angle to cause high misses. The only other thing, other than equipment failure, is the distance to your target is shorter meaning your zero is longer than the distance you are shooting and you are hitting high.

Let's try break it down. At 100 yards with an elevation of 10 feet (3 yards) the angle would be roughly 2-3 degrees. That is a Cosine of 0.99 so the actual distance is 99 yards. Given that and knowing that the .17hmr is 1/2" - 3/4" high at 50 yards we know that any discrepancy between your 100 yard zero and actual distance of 99 yards is negligible.

Now you say a rabbit popped out at 90 yards. Knowing the discrepancy beetween 100 yards and 50 yards, and between line of sight to actual distance we can now safely say that the rabbit was basically in your point blank range. Therefore any misses would be down to:

• The distance to your target is wrong
• The height is far, far greater
• There may be an issue with your kit

There is always the very simple solution of a miss. IOW you simply missed. No miscalculation of distance, no fault with your kit. Given your zero is still spot on it could be "just one of those things".

everypenny

Or the rabbit had a forcefield!!!

sniperman

Cass wrote: »

Nearly forgot the OP. Sorry lad.


It does relate to the posts though.

• Are you sure of the actual distance you are shooting at?
• Is the height of the ground only 10 feet?
• Have you a range finder or other measuring device?

If the height is only 10 feet then it's not enough of an angle to cause high misses. The only other thing, other than equipment failure, is the distance to your target is shorter meaning your zero is longer than the distance you are shooting and you are hitting high.

Let's try break it down. At 100 yards with an elevation of 10 feet (3 yards) the angle would be roughly 2-3 degrees. That is a Cosine of 0.99 so the actual distance is 99 yards. Given that and knowing that the .17hmr is 1/2" - 3/4" high at 50 yards we know that any discrepancy between your 100 yard zero and actual distance of 99 yards is negligible.

Now you say a rabbit popped out at 90 yards. Knowing the discrepancy beetween 100 yards and 50 yards, and between line of sight to actual distance we can now safely say that the rabbit was basically in your point blank range. Therefore any misses would be down to:

• The distance to your target is wrong
• The height is far, far greater
• There may be an issue with your kit

There is always the very simple solution of a miss. IOW you simply missed. No miscalculation of distance, no fault with your kit. Given your zero is still spot on it could be "just one of those things".

thanks cass,gonna get a range finder for distance soon,how i measrued my zero distance was a long peice of fly fishing line cut to 25 yards spaced out 4 times,thought that would give me a good idea,distances to the rabbit shots was me guesing,so "The distance to your target is wrong"could be the reason,will be able to get it right with the rangefinder,hopefully,thanks;)

Cass

What, like a Jedi Rabbit?

everypenny

Thank god you didn't decide to show the rampant one!

bravestar

everypenny wrote: »

Sorry, missed that!


If you really want to go into it then cast your mind back to maths in school and all that trigonometry that you never thought you'd ever use.


Every angle has an equivalent cosine. If you don't believe me then here's NASA backing me up
http://www.grc.nasa.gov/WWW/K-12/airplane/tablcos.html


The COSINE is a great bit of gear. You can use it to figure out your horizontal distance to a target. All you do is multiply your Line of Sight range (say 100 m) by the COSINE of the angle you're shooting at.


Lets say its 45 degrees. As you can see in the table above the COSINE of 45 degrees is .7071, which we round up to .71
Multiply your range (100) by this and you get what you should actually be holding for that target/angle which is a range of 71 meters. Make sense?


Working out angles can be very tricky, and obviously you can't guesstimate, especially at longer distances where an error can turn in to meters off target!
But luckily there are a few products on the market. One that I like (and if anyone wants to buy me one for a present feel free to PM me) is an Angle Cosine Indicator.
http://accurateshooter.net/Blog/wardaci01.jpg


You can get one with just angle, just cosine or both. They're mechanical and so don't need batteries and you can attach them to a picatinny rail on your rifle.


Then all you need is a beautiful mind or a phone calculator to do the math.


Here's a good video on it.
https://www.youtube.com/watch?v=95bPADBjkbI

I like the redneck solution, here's one I made about a year ago...

Aim through the straw, read angle, then do the maths.

everypenny

Perfect! Is it accurate at long range distance?

Whats the green rod for?

I'm trying to come up with a way to do a good range card for different distances at different angles. But there's too much data to insert.

Any ideas?

bravestar

everypenny wrote: »

Perfect! Is it accurate at long range distance?

Whats the green rod for?

I'm trying to come up with a way to do a good range card for different distances at different angles. But there's too much data to insert.

Any ideas?

The green rod is a straw I cut. Aim through that at the target, then pinch the string in place and move it away from your face so you can read the angel. I've only used it out to 600metres with my .223. It's worked perfect for that.

Re: a range card for angles, check my attachment here, I can email you a higher quality PDF.

bravestar

Sorry, attachment wouldn't work in last post.

everypenny

Sweet! Thanks a mil..

hiddenmongoose

If your really honestly shooting at distances and angles that are big enough to make an EFFECTIVE difference then get a cosine angle indicator .heres one on my 308.

I use it when lr rock bashing up the mountains .It is simple as it gives you a readout of the cosine of the angle your rifle is pointing at.
And instead of multiplying the cosine of the angle by the range ie .9 * 100m =90 m aka the riflemans rule its more accurate to multiply the cosine by your dope/clicks/drop .9 * 1.0mil =0.9mil aka the improved riflemans rule.
As cass said I reckon the op has incorrectly judged the distance .

everypenny

hiddenmongoose wrote: »

its more accurate to multiply the cosine by your dope/clicks/drop .9 * 1.0mil =0.9mil aka the improved riflemans rule.
As cass said I reckon the op has incorrectly judged the distance .

Great idea!

Where did you pick up the indicator? I've been looking for one in Ireland but I cant find one.

hiddenmongoose

everypenny wrote: »

Great idea!

Where did you pick up the indicator? I've been looking for one in Ireland but I cant find one.

http://www.snipertools.com/ Got it several years ago.When you use it regularly you get to see its not actually needed for most everyday stuff,and shots you would think you need to hold for you dont really need to either.That said when Im up the hills its also amazing how much you can be off without allowing for it

FISMA

sniperman,
You were shooting a .17hmr, correct? Were you shooting freehand, off sticks, bench, bipod, or something else? When you went from level to check zero to the hill, did you use the same technique? Or for example, did you use a bench to zero and bipod for the bunnies?

The effect you are discussing is well known to hunters and shooters. Aim low, is what we say, both uphill and down.

If I am reading this correctly, you are saying that the hill you were on was about 10.0 feet above level ground, on which the bunnies were 270. feet downrange.

Regardless of whether you gave the horizontal distance (adjacent) or the actual line of sight distance (hypotenuse), you are still talking about a small angle of 2.12 degrees. That is not a considerable angle to shoot from and would not be my first suspect. The effect we are talking about here is usually for steep hills/mountains.

If you had a solid firing position, I would put my money on an incorrect target distance or incorrect bullet-reticle reading, as mentioned by Cass.

Do you holdover or dial in? Without a range finder, I assume you hold over.

If so, you need to understand that those marks on your scope's reticle are general. Maybe designed for a centerfire round as well. No one knows what your gun and ammo are going to do at 100, 150, and 200ya, except for you.

BTW - what reticle have you?

I'll use this chart as an example.

Let's just say that the reticle on the left hand side is the one that you have found for the 17hmr zeroed at 100ya.

The second crosshair does not get you spot on at 200ya. The "-3.1" tells you that you will be 3.1" low at 200ya.

You should do a bit of research to find your reticle and round to determine the expected poi. Then, get out and test.

Depending on your scope, the manufacturer may have a nice ap, like Nikon's Spot On Ballistic Match Technology to help.

sniperman

hi fisma,thanks for the write up.im using a hawke varmint 6-24x44,on my 455 cz hmr 16" heavy barrel.i use sand bags to zero,but when i was on the bit of a hill,i used my coat as a rest, i was very steady,so like cass said said,i think i may have miss read distance,but even with a give or take of 10-20 yards,i did not think it would matter since the hmr is supposed to be flat from 50-120 yards??ill just have to get a range finder and do some tests,thanks;)

Mackcon

I clean my hmr every 50 bullets or so , there a dirty little round and can cause 2-3 inch fliers , may be the cause

sniperman

Mackcon wrote: »

I clean my hmr every 50 bullets or so , there a dirty little round and can cause 2-3 inch fliers , may be the cause

hi mac,do you use any solvents,or do you just dry patch it?

Mackcon

sniperman wrote: »

hi mac,do you use any solvents,or do you just dry patch it?

I use hopes bore solvent and give it a good clean , I used to dry patch it and it still screwed with my poi after a few shots so I don't chance it now

juice1304

Mackcon wrote: »

I use hopes bore solvent and give it a good clean , I used to dry patch it and it still screwed with my poi after a few shots so I don't chance it now

Hoppes is a pretty heavy solvent, i wouldn't be cleaning a rimfire with it or maybe once a year, but certainly not after 50 rounds. The length of your barrel life is going to be dramatically reduced. I wouldn't even clean my .308 with it that often I just use a very mild solvent to clean the carbon out until i see a noticeable change in accuracy due to copper build up.

Mackcon

juice1304 wrote: »

Hoppes is a pretty heavy solvent, i wouldn't be cleaning a rimfire with it or maybe once a year, but certainly not after 50 rounds. The length of your barrel life is going to be dramatically reduced. I wouldn't even clean my .308 with it that often I just use a very mild solvent to clean the carbon out until i see a noticeable change in accuracy due to copper build up.

What would you use in a mild solvent

juice1304

I use M-Pro7 It is a bit more expensive but it lasts and dose a fantastic job. It is non toxic too.

juice1304

https://www.youtube.com/watch?v=QQKhlo2Ffug

zen260

once you are shooting at a 45 degree angle or over,be it upwards or downwards you will always shoot high as apposed to flat shooting,,,

Mackcon

juice1304 wrote: »

https://www.youtube.com/watch?v=QQKhlo2Ffug

Thanks pal il give it a bash

German pointer

sniperman wrote: »

hi all,was out with me hmr yesterday to check zero at 100 yrds,all was fine,groups within 1",so as i was out, decided to go to a sand quarry i have permission on, to have a few shots at rabbits,went up on a bit of a hill looking down,about 10 feet up, so im now shooting downward.first rabbit popped out about 90 yrds,bang,about 2" over head,same with the next 4,thought something wrong,went back down to level ground to check zero again,she was bang on,,so my question is would POI change form shooting level 1"groups to shooting 2" high downwards,any thoughts/advice welcome,

Have a look at the link below for a real life test

http://www.youtube.com/watch?v=0mygTqP88FA

hiddenmongoose

zen260 wrote: »

once you are shooting at a 45 degree angle or over,be it upwards or downwards you will always shoot high as apposed to flat shooting,,,

Not as simple as that.It doesnt take as big an angle.It depends on distance and angle .And then you must consider how flat your calibre is and what the danger space/allowence for error you can have on your target as to whether its of any importance.

A shot at 600 meters at 25 degrees (cos 0.906) equals 543m .Now thats a difference of 60 meters .at 600m for my 308 I need 3.9mils ,for 540 I only need 3.4mils thats .5 mils of a difference using the riflemans rule..5 mils too much at 540m equals being 27cm high.
Same distance same angle with the improved riflemans rule is 3.9 mils by .906 equals 3.5 mils dialled for that shot.

lefthooker

This is why I think a rangefinder with built in angle compensator is worth having. Its easy to misjudge distance when shooting up or downhill.
After checking zero on my .204 I went to shoot some steel. I placed a steel plate on a headland and walked 100 paces back up the field. Whipped out my Leica but it only read 94yds. Corrected with another 6 paces. After that stepped another 100 paces out to what should have been 200yds but the rangefinder only had 192 so needed another 8 paces. So what on the flat should have been 200yds in reality was only 186. Not a big deal cos the .204 shoots so flat and is zeroed @ 200yds. But that's an uphill rise of around 75yds, someone else can calculate the degree for me.

extremetaz

everypenny wrote: »

There is a big reward if you're shooting out to a big distance or from a large angle though. Shooting at 100 meters from only a 25 degree slope changes the range to 90 meters. (COS of 25 degrees =.9)

I wasn't disagreeing with you - What I was saying is that it's is a *VERY* good way of making that particular calculation accessible and easy to remember and perform.

dogcityroller

I use strelok for my android phone. It has a built in angle cosine. You just put the phone on the barrel and it works out the percentage for you. I shoot on fairly flat ground so I don't have much use for it in my 223. For shooting squirrels withe the air rifle it can come in handy to plot out areas I'm shooting in. You wouldn't believe some of the allowances you have to make when tour aiming at the tops of trees from the ground.