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Thread: Assessing resulting vertical spread from ES (or SD) for your load at distance

  1. #1
    Member Puffin's Avatar
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    Assessing resulting vertical spread from ES (or SD) for your load at distance

    How are the long range shooters amongst us determining the vertical dispersion that results from a given ES (or SD) at distance in their loads ?

    It is clear that if using the usual shooting apps for this that the sight height should first be set to zero, and the zero distance also set to as close to zero as possible, as this approximates a horizontal launch. Then the difference in drop (from the horizontal) for variations in entered muzzle velocity will be correctly translated into vertical POI change as would occur in practice for ES variations.
    If on the other hand the usual zeroing distance of 100m or 200m is entered into the ballistic app instead for this type of simulation, then the reported change in the vertical point of impact for a given variation in MV will be reduced, since the angle of launch is recalculated each time to match the zero distance, rather than being held constant.

    I know this may sound a bit cryptic, but anyone who has looked into this will probably know what I mean.
    So what method/s did you use?
    Bagheera likes this.

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    Member Puffin's Avatar
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    ….. a worked example:

    Let’s say a bullet is being used with an average muzzle velocity of 2800 ft/sec. The ES of the load in that particular rifle is known to be 10 ft/sec, and the interest is in the maximum amount of vertical spread this could end up producing on a target at 1 kilometre.

    The Applied Ballistics or JBM balllistcs calculator is set up with all the necessary inputs (including the usual zero distance of say 200m) and the drop is found for a 2800 ft/sec muzzle velocity. Lets say it is exactly 10 metres. Then the MV is changed to 2790 ft/sec (and 2810ft/sec, just to confirm), and new POIs at those settings are calculated and found to be say 70mm above and below the nominal. The ES is expected then to produce a 140mm spread on the target.

    In fact the true dispersion from that size of ES is likely to be around 85mm as follows: The calculations are repeated, except this time the zero distance is moved back in the simulation to as close to the muzzle as possible (ideally at the muzzle, but 1 metre may be the closest that can be entered). Because of the short zero it is necessary to also reduce the sight height to zero so the rifle doesn’t end up being tipped at a steep angle in the simulation. This then simulates a barrel held perfectly horizontally and where the bullet drops away below this line. More importantly with these settings the barrel remains in exactly the same position for both the 2800 ft/sec and the 2790 ft/sec and 2810 ft/sec calculations, giving a 85mm difference in POI..
    In the first example where the zero was 200m this is not the case: the barrel is simulated to launch the bullets at very slightly different angles for each of the three muzzle velocities, having the effect to partially compensating for the difference in MV.

    The error using these ballistics programs to find vertical spread from ES becomes more pronounced the further out the zero distance used. Take an extreme example where the zero entered into the ballistics program is right out at the target at 1 km. The simulation will clearly calculate no change in the POI for any variations in muzzle velocity !

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    Your thinking way to hard about it, just go shooting
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    Member 199p's Avatar
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    If your getting a large difference try factory ammo and see of it dose the same or buy a fancy crono


    Sent from my SM-G935F using Tapatalk
    BRADS likes this.
    Konus binoculars " The power to imagine"

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    One day when I grow up I hope I will be smart enough just to understand the question. Being able to add anything intelligent is just too much to wish for. I am very lucky I have a chum who understands this stuff so I don't have to.

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    Sending it Gibo's Avatar
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    Way over my head. But wouldnt an ES of +- 10 fps be an ES of 20?

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    Whats the question?

    You technically should calculate all velocitys/ES at your zero range. E.g the ES at the target. Not impossible to do with the right chrono

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    Member Puffin's Avatar
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    Quote Originally Posted by longrange308 View Post
    Your thinking way to hard about it, just go shooting
    " Long range shooting is all about limiting your uncertainties. The more variables you know and understand, the less you have to predict and the more certain you will be of impacting the target "

    Quote Originally Posted by 199p View Post
    If your getting a large difference try factory ammo and see of it dose the same or buy a fancy crono
    Sound advice for shooters without a fancy chronograph or tight ES.

    Quote Originally Posted by Gibo View Post
    Way over my head. But wouldnt an ES of +- 10 fps be an ES of 20?
    I like using the ALT+0177 keyboard shortcut (number lock has to be on)

    Quote Originally Posted by PerazziSC3 View Post
    Whats the question?
    Methods of determining the vertical dispersion at distance resulting from a given ES.


    Perhaps of interest: back in the day I prepared a trajectory simulator in VisiCalc, Lotus123 or somesuch that started out calculating drop from the horizontal before adding in elevation for the zero distance (the formulas modelled published Sierra and Hornady data) so that would have been perfect for this sort of task.

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    Hi puffin, I really hadn't thought about this but it is true !

    A better way to explore it would be to model a horizontal launch by setting the sight height to approx your own and specify a "zero offset" of exactly the same,using a short range . I'd use a zero range of 10m not sure why, dont trust percentage rounding errors which could get out of control at micro ranges orders of magnitude less than your working range eg drop difference over the ES could be like a mini fraction of a millimetre and just rounded out.

    The answer Is Brian Litz Applied Ballistics Analytics program which will simulate hundreds or thousands of shots at working ranges using your specified ES (actually you input the sd). You do need a windows machine and it costs about $200 . In his book accuracy and precision he models the relative benefits of improving ES in several scenarios. He doesn't discuss the error you explained here when using standard ballistic trajectory programs but he must have dealt with it to get the analyses he wanted.
    Puffin likes this.

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    This is a good idea, I hadn't considered looking at it like this, I ushally just measure groups and worked off that.

    Just played around with a online calculator ShootersCalculator.com | Ballistic Trajectory Calculator not sure how accurate it is but it let's you enter sight height at 0 and zero at 0.

    For my loads with a maximum spread of 25fps it gives me a different point of impact of 1.3 inches at 500, which is somthing to think about , maybe we should be using the bottom number of ES or lower third instead of average to build drop charts, as I would ushally prefer to have point of impact slightly high than low.

    If i built my drop charts off my avg velocity i would be .78inch low (20mm) @ 500 if my load was at the bottom of my ES. Not to much really at 500y, but I could see it being interesting at 1000y, could easily be a miss depending on target size.

    I guess the goal is to keep ES down.
    Puffin likes this.

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    Member Micky Duck's Avatar
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    its all there in "the sharpshooter" written years and years ago...... and they used four rifles from memory to try it out. mind you back then 50fps variation was very good and they were still shooting deer out to 600 yards.

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    ASSUMING you are shooting a 3 shot group at the same speed and another at same speed and using group centres to get the -/+ 85mm it MIGHT make sense BUT if you are using single rounds to make calculations you arent taking into account any verticle error that normal group size will add /subtract to the maths....go shoot something LOL

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    I'm guessing that's a book? I am unfimiliar with it, I googled it and came up with a 22lr reloading tool, didn't know that existed, I'm learning lots today.

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    Member Micky Duck's Avatar
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    also reading your first post again it doesnt matter stuff all if barrel is level or not the AMOUNT of drop or the difference between shots will be the same no matter what angle barrel is on within reason......like 45 degrees is obviously going to be different to 90 degrees but 5 vs 10 wont be buggerall as increase of gravitational pull will be so small as to be unmeasurable.

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    Member Micky Duck's Avatar
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    Quote Originally Posted by rusl View Post
    I'm guessing that's a book? I am unfimiliar with it, I googled it and came up with a 22lr reloading tool, didn't know that existed, I'm learning lots today.
    sharpshooter was considered the "bible" for longer range shooting of game and getting rifles to behave back in the day... Matt and Bruce Grant are the authors. it still worth a read as it contains many truths which people seem to forget.
    rusl and rewa like this.

 

 

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