Your thoughts on elevation scenario

[DaveT]

A little stuck to explain the 100/200 yard come-up differences so any thoughts appreciated......

 

Scenario.......

 

6mm SLR chambering zeroed at 100 yards with 115g DTAC (Boron coated) & 43.5g of H4831sc fired in temperature of 75f goes over my Magnetospeed chrono at 2875 FPS.

 

Using Applied Ballistics app (usually very accurate I have found) the come-up for 200 yards indicates 1.5MOA.....in the same conditions as zeroed it actually only needs 1.0 MOA. (to get Applied Ballistics to return that figure it would have to be going at a MV of 3300 FPS...its not!)

 

I kept the rounds shaded so expect the powder temperature to be less than ambient (assumption is 65f)

 

I have not calibrated the scope (Sightron nearly new) but doubt it would be that far off to account for the MOA difference between APP Ballistics and actual results in the field.

 

At the other end of the distance scale I have shot it at 900m / 984 yards and needed 27.5 MOA at 65f.

 

I am probably having a 'brain-fart' and put something odd into the App but can't explain the discrepancy.....no doubt after sleeping on it I will have the 'AHA' moment but help gratefully received!

 

 

[jcampbellsmith]

Snap. I have the same issue. Regards JCS

[DaveT]

It predicts the 984 yard come-up almost precisely so why does it struggle with the close stuff?

 

Maybe a call to Mr Litz is needed?

[oaken]

Have you input your scope height correctly?

[jcampbellsmith]

No, but correcting the scope height to 1.8 inches makes very little difference. The 175 yds drop is still too much.Regards JCS

[Gandy]

I have had something similar and a post on here ages ago solved it.

 

The ground where I shoot and used to prefer to zero is best described as a crucible surrounded by trees. When having a card made by our in house Litz the elevation elements did not tally. I just used to correct.

 

Then baldie, who knows his rifles had the same issue with the same cards. His zeros were arrived by shooting at Diggle. Gun pimp, who shoots widely and is the gaffer at Diggle advised of the vertical wind element affecting elevation.

 

I've found this affects my situation and went out in light and strong winds to test my theory which seemed to meet my thoughts.

 

Hope this helps.

 

Gandy.

[tisme]

I have the same thing with Strelock my 200 - 300 is wrong (high) but anything past 600 is very good out to 1000yards.

[brown dog]

A little stuck to explain the 100/200 yard come-up differences so any thoughts appreciated......

 

Scenario.......

 

6mm SLR chambering zeroed at 100 yards with 115g DTAC (Boron coated) & 43.5g of H4831sc fired in temperature of 75f goes over my Magnetospeed chrono at 2875 FPS.

 

Using Applied Ballistics app (usually very accurate I have found) the come-up for 200 yards indicates 1.5MOA.....in the same conditions as zeroed it actually only needs 1.0 MOA. (to get Applied Ballistics to return that figure it would have to be going at a MV of 3300 FPS...its not!)

 

I kept the rounds shaded so expect the powder temperature to be less than ambient (assumption is 65f)

 

I have not calibrated the scope (Sightron nearly new) but doubt it would be that far off to account for the MOA difference between APP Ballistics and actual results in the field.

 

At the other end of the distance scale I have shot it at 900m / 984 yards and needed 27.5 MOA at 65f.

 

I am probably having a 'brain-fart' and put something odd into the App but can't explain the discrepancy.....no doubt after sleeping on it I will have the 'AHA' moment but help gratefully received!

 

 

 

Half an MOA............ light condition changing from bright to overcast (or vice versa) will cause a vertical 0.5MOA shift on its own - before you even get to vertical wind component.

 

Think back to setting realistic real-world non lab condition tolerances - you have a data point that is only 0.5 MOA out!

[tackb]

You could try a zero at a greater distance ? It's very easy to get your 100m zero out a fraction which will increase the error as you extent the range whereas if you get a good mpi at 300 or 400 (wind and weather permitting) and zero to this when you come back to your closer zeros the error will reduce and by your own admission the longer range calcs are already close ?

 

Just a thought?

[jcampbellsmith]

You could try a zero at a greater distance ? It's very easy to get your 100m zero out a fraction which will increase the error as you extent the range whereas if you get a good mpi at 300 or 400 (wind and weather permitting) and zero to this when you come back to your closer zeros the error will reduce and by your own admission the longer range calcs are already close ?

 

Just a thought?

Thanks, good point. Going through my records, I seem to have a recurring issue with 100 yd zero and 175 yd zero not matching up. I'm fairly sure in the past I've set the 175 yd zero and then been out at 100yds.

 

Regards

 

JCS

[gbal]

Hmmm....this is an interesting conundrum.

At least four savvy shooters on here have noticed this effect,though it is interesting that it isn't widely reported-perhaps because shooters sight in at 100/200 and just don't check fall of shot carefully elsewhere?

Nor is it discussed by accuracy nuts in 100/200 Bench Rest-though they don't need to worry about a half moa vertical POI change.The 300 for score would,but are sighted in for 300.So we can't access the usually vast data base from that sort of precision competition.(Vast matters because of statistical issues-3shots won't really do,nor 10 if we want a sound answer-it will for the individual field shooter,though!)

 

Pragmatically,does it matter-well,perhaps not-given you know it is there you can allow for it (if it is there and you don't know,you may be out around 1/2 moa),which is unlikely to be crucial outto say 250y.

 

What would be interesting,and relevant is to know just where the actual POI and predicted poi come together....we know it is very close at 900+ for Dave,but just where did it start-be nice if it was 300y,much less so if it was 750y...

 

One 'solution' might be to zero at the modal (most likely) shooting distance,then any error is minimised-within field distances that should be reported. But that isn't an explanation in itself.

So what are the options (some already mentioned in posts)

 

1) errors in measurement of the field data input -1/8 moa has a cumulative effect-but not that big

 

2) errors in the other input data-eg scope height,but as above and already have some indication of effects for scope height,and velocity variations...don't seem enough

 

3) local factors,not really assessable,or allowable in program,such as wind irregularity vertical

 

4) program has inbuilt assumptions/approximations that do not match individual shooter data-almost certainly so,but likely again to be quite small (BC eg is not a constant,but is not likely to be out enough at 200y-check the small print in the program for any other assumptions.)

 

All these are likely to be small effects,possibly cumulative (your 1/4 moa tackdriver at 100y will not be a one moa rifle at 400y).They may even cancel out-one of the real higher order complexities of ballistics (life in general ) is that factors may not be linear in their effect,and 'worse' yet,may be interactive with other variables (eg effect goes in one way under some conditions,reverses for others...(heavy gym work enhances young athletic performance,completely knackers the elderly,to take but one personal example.

Accuracy and velocity nodes might be a more compelling and relevant example,velocity increases reducing but the increasing groups,and where the amount of change varies too,around different nodes!)

 

So....it would be helpful to have more information about zero testing conditions (Vince's point about vertical wind effect (and not just head/tail wind effects) is a contender-especially over uneven 'field' ground,so testing in zero wind ideally over flat rifle range surface should give an indication of it's effect....

 

As with all puzzlers,it would be good to know why.

 

 

Matt-I can't find much on atmospheric brightness effects-other than the USMC combat marksmanship centre (quantico) 2008 lecture on weather effects.But this is about open combat sights(and essentially effects of dull/bright on range judgement errors,or front sight blade reflection errors),none of which would be relevant to scope use with known ranges.Is there yet another factor to add to the leaky raft of accuracy vectors?

 

Gbal

[1967spud]

Dave what twist barrel on you slr?

[Gandy]

Half an MOA............ light condition changing from bright to overcast (or vice versa) will cause a vertical 0.5MOA shift on its own - before you even get to vertical wind component.

 

Think back to setting realistic real-world non lab condition tolerances - you have a data point that is only 0.5 MOA out!

 

I would listen to this.

[jcampbellsmith]

Thanks for all the comments. One key point emerging for me is my record keeping isn't good enough. I need to do some further testing from 100 - 500 yds, but my favourite testing spot is 230 miles away.

 

Regards

 

JCS

[DaveT]

Gents

 

Just managed to get online and am reading your comments.

 

Many thanks for all your inputs...I will chew them over this evening and reply as necessary.

 

BD....agree that its not a gross error and I always do field tests anyway...its just that the app is normally so damned close that's thrown me on this one. The 100 yard zero and 200 yard field test were done in exactly the same conditions and winds were very light ....still its what actually happens that counts! Somehow I just thought these apps would have an easier time predicting close range come-ups than at the longer pokes.

[DaveT]

Have you input your scope height correctly?

Rup...believe so but will check anyway.

[DaveT]

I have had something similar and a post on here ages ago solved it.

 

The ground where I shoot and used to prefer to zero is best described as a crucible surrounded by trees. When having a card made by our in house Litz the elevation elements did not tally. I just used to correct.

 

Then baldie, who knows his rifles had the same issue with the same cards. His zeros were arrived by shooting at Diggle. Gun pimp, who shoots widely and is the gaffer at Diggle advised of the vertical wind element affecting elevation.

 

I've found this affects my situation and went out in light and strong winds to test my theory which seemed to meet my thoughts.

 

Hope this helps.

 

Gandy.

Gandy...I have experienced the vertical effect of cross-winds in the past but it tops out at about .030 MOA per MPH of wind (FV wind at that ) ...on the day the winds were very light ...maybe 5 mph at most.

[DaveT]

You could try a zero at a greater distance ? It's very easy to get your 100m zero out a fraction which will increase the error as you extent the range whereas if you get a good mpi at 300 or 400 (wind and weather permitting) and zero to this when you come back to your closer zeros the error will reduce and by your own admission the longer range calcs are already close ?

 

Just a thought?

And a good thought at that ...however I don't feel that I can get a rock-solid zero at those distances due to wind etc plus my own 'driver-error'.

[DaveT]

Dave what twist barrel on you slr?

1:7.5.......Border cut barrel so hopefully accurately rifled.

 

No apparent stability problems with 115 DTACS

[DaveT]

I also plan to continue the creation of my drop tables from 300 to 1000 via field tests and will see if the app suddenly agrees at some set distance.

 

In any event it should get me 'on paper' well enough at the various ranges before I get actual drops.

 

It all takes so much time!

[brown dog]

Gandy...I have experienced the vertical effect of cross-winds in the past but it tops out at about .030 MOA per MPH of wind (FV wind at that ) ...on the day the winds were very light ...maybe 5 mph at most.

 

What Gandy's referring to is the vertical vector component of a wind, not the lift effect of a crosswind (eg you're shooting at a mountain face and the wind is blowing down the mountain - it has a horizontal (in your face) component, but your bullet will also experience the vector component of the wind that is vertically down, and will be blown lower than you expect.

If you estimate the vertical component, treat it in exactly the same way as you would for a crosswind (ie, in effect tip your wind table on its side, work out the deflection for this vertical 'crosswind' ........but apply it as an elevation change).

[DaveT]

BD....see what you mean now.

 

The range is on an estuary so pretty flat but your point understood.

 

I think in this case I just have to accept that the theory conflicts with reality ...its simply a means to get me in the ball-park.

[brown dog]

I think in this case I just have to accept that the theory conflicts with reality ...its simply a means to get me in the ball-park.

 

The theory will be correct; what you're seeing is either the effect of an environmental variable that you're unaware of/not accounting for (such as a vertical wind vector) or some element of your data input is flawed (GIGO / SISO)

 

Ballistic prediction is so absolutely precise nowadays that the fault will lie with either the 'unseen' variable or your data entry - ie if you're finding a predictor of the quality of Litz's is 'wrong'; it's actually you

[Murph]

I have the same thing with Strelock my 200 - 300 is wrong (high) but anything past 600 is very good out to 1000yards.

Have the same problem with mine aswell. Solved it by using 2 seperate speed setting. For example for 53gr vmax out to 450 my fps is 3450 and from there to 750 the speed is set at 3100. I use strelok pro which allows you to set different speeds for whatever temperature and atmospheric conditions you are shooting in. The heavier the bullet I use the less discrepancy in speed I have to use.

[tisme]

Have the same problem with mine aswell. Solved it by using 2 seperate speed setting. For example for 53gr vmax out to 450 my fps is 3450 and from there to 750 the speed is set at 3100. I use strelok pro which allows you to set different speeds for whatever temperature and atmospheric conditions you are shooting in. The heavier the bullet I use the less discrepancy in speed I have to use.

This has got me thinking. Sierra bullets give different BCs at different speeds. It could be the BC needs to be higher for the first few hundred yards.