How Pellet Design Can Affect Group Size

Discussion in 'General Airgun Chat' started by Ballisticboy, Dec 3, 2019.

  1. Ballisticboy

    Ballisticboy Active Member

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    One of the factors affecting group sizes is the design of the pellet itself. Pellet shape and design affect more than just the drag and velocity drop, they also affect how it flies through the air and how it reacts to different stimuli.

    Suppose you are the perfect shooter using the perfect gun. Each shot you fire you control the recoil exactly the same as the last one and you aim the gun at exactly the same point. The gun also behaves in exactly the same way every shot. You still will not get every pellet flying through the same hole in the target due to small variations in the pellet size, construction and shape. The pellet design will control how the pellet reacts to these small variations and the size of the resulting group.

    When a pellet leaves the barrel of a gun it will not leave perfectly. The pellet may be pointing at a small angle to the line of the barrel due to being for example slightly asymmetric, known as a yaw angle, or, more commonly, the pellet may be pointing in the line of the barrel as it leaves but will immediately start to change the angle in which it is pointing due to many different things, both in the gun and on the pellet outside the gun. The speed at which the angle changes is known as a yaw rate. How the pellet reacts to the yaw or yaw rate is what is decided by its design and what decides the resulting group size.

    There are two ways of achieving a small group size with a pellet. One is to make your pellet to a very exact size and shape and concentrate on manufacturing each and every pellet to that exact size and shape. This will give a very consistent launch from a rifle provided the pellet is a good match to the barrel. Matching the pellet and barrel dimensions coupled with exacting tolerances in the pellet shape, centre of gravity etc. will produce very small yaw angles and rates, which will mean that each pellet will fly in the same repeatable way. The problem with this method is that as dies wear or if the pellet dimensions are not an exact match to a barrel yaw angles and rates will increase and with them the group sizes.

    The second method is to produce a pellet design which concentrates on reducing the effect of the yaw angle and rate. In this case, instead of trying to minimise the yaw angle and rates, you accept that they will exist and minimise their effects on the pellet trajectory, you make the pellet more tolerant of errors. For example, suppose you had two pellets which both came out of the barrel with the same yaw angle or yaw rate, the pellet designed to be more tolerant of any type of yaw would produce the smaller group. There will still be some benefits from exact manufacture and barrel pellet matching but the benefits will be reduced. The advantage is that with the yaw tolerant design you potentially reduce the need for pellet barrel matching to achieve a certain group size.

    Using a pellet designed by the second method does not mean the ultimate performance will be improved compared to one designed by the first method. If there is no pellet yaw angle or rate then there is nothing to improve upon until the dies start to wear etc. There is also the danger that, in making a pellet which is more tolerant of launch errors, you may well increase the pellet yaw angles and rates as it leaves the barrel. In such a case the group size may be bigger or no better than that from a pellet which has not been optimised in the same way.

    The conventional diabolo pellet shape is not a good ballistic shape from the point of view of minimising the effects of yaw angles and yaw rates. Small group sizes are achieved through precise manufacture and barrel pellet matching. As soon as there are relatively small amounts of yaw in any form the group sizes will start to grow. The stability problems associated with pellets at high speeds and longer ranges are due to a different set of design properties, linked to but not solely caused by the same problems as the sensitivity to yaw angles and rates.
     
    gdavison, slippschnitz, DeanB and 2 others like this.
  2. Yorkshiretea

    Yorkshiretea B Grade Bandit

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    Is the perfect repeatable design possible using Swage and Lead BB?
     
  3. martin blue eyes

    martin blue eyes shoot, pray and hope

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    Thanks for the very informative information, now I have a good excuse for being a crap shot lol, but in all honesty it makes you wonder how we hit anything with so many variables.
     
  4. GraySaint

    GraySaint WFTF 2015 World Champion

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    Can you please describe an instance of such design, for clarification?
     
  5. hmangphilly

    hmangphilly Can't Re Member

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    Does this mean we are wasting out time trying to achieve even and concentric crowns ?

    The theory being that an uneven crown will promote exactly this type of yaw effect
     
  6. Dale

    Dale Active Member

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    I would suggest the answer is no, you are not wasting your time.

    I would read this to indicate that as such variables will inevitably occur, reducing anything that can cause any further influence will be beneficial.


    Perhaps despite the variables, unless something is way off, what we have actually works relatively well.
     
  7. RobF

    RobF Administrator Staff Member

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    I think it makes it more important. If current pellet shapes are poor at coping with it then you need a crown which is even and concentric to ensure it gets the best exit as possible.
     
    subaru swift and Ballisticboy like this.
  8. JerryD

    JerryD Active Member

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    Taking BB's point about the diabolo shape being poor, and the fact that pellet manufacture can vary resulting in a previously "good" rifle suddenly grouping like a shotgun when going to a different pellet batch, what would be the best pellet shape for a sub 12 ft-lb projectile to be more tolerrant of yaw?

    AFAIK the "boat-tail" shape is more ideally suited to supersonic ammunition - how would a "perfect" pellet look, and what would that do to barrel diameters?



    .
     
  9. Ballisticboy

    Ballisticboy Active Member

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    I do not know of any airgun pellet designs which use the aproach of reducing pellet sensitivity to yaw. It was the approach we used when carrying out the experiments in pellet design described in an earlier thread.

    There is no single answer to this question, the approach I took years ago was just one solution. There are a lot of variables to consider both aerodynamic and mechanical. Combine this with the low twist rates on most rifles and the desire to keep pellet mass low in a 12FPE application and the problem becomes more difficult. But it is not impossible, the answer is in geting the correct combination of variables to give the result you want to achieve. Gaining shooter acceptance of something which is not a diabolo may be the hardest problem.
    A boat tail is actually at its most effective at subsonic speeds just below Mach 1. At supersonic speeds the relative effectiveness of a boat tail decreases rapidly as the speed rises. Boat tails can give problems with inducing yaw as a bullet leaves the barrel, cylindrical bases can be better for short range groups.
     
    Last edited: Dec 4, 2019
  10. Fly fisher

    Fly fisher Member

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    Last Sunday I shot in a temperature of 8 degrees C. The wind was NNE 9 mph gusting to 15mph. Of the 30 targets I shot at I knocked over 26. The pellets were JSB 10.34 straight from the tin. Obviously my rifle is totally stupid and has no understanding of pellet design or ballistics.
     
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  11. Brian.Samson

    Brian.Samson Allowed in Sales Staff Member

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    Try it again with Hobby's and see how you do - then come back and tell us your thoughts on whether pellet design played a part or not.
     
    Tartan, Gary Martin and old foggy like this.
  12. RobF

    RobF Administrator Staff Member

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    In reality having done several blind tests with shooters and anemometers shooters vastly over estimate the wind speed, often lead by weather predictions which are designed to predict a meter 10m up a pole clear of any obstruction for 1oo m radius. Normally over open ground you're lucky to see 1/2 what people think the speed is, in woodland it can be 1/4 or less.
     
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  13. Conor

    Conor Never been banned from sales Staff Member

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    Quite correct, I’ve heard many exaggerated wind statements over the years. It’s all down to feel, experience calculated guesswork and a dash of luck.
     
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  14. Welsh Wizard

    Welsh Wizard FT: You Give, You take! Staff Member

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    I've never considered wind as a speed, only a strength.
    It's measured in half dots from 0.5 to 2.5 on my scope.
    0 - 0.5 is no real wind but the pellet moves up to 20mm.
    0.5 - 1 dot is a breeze
    1 to 2 dots is windy.
    2.5 dots is very windy!
    More than 2.5 and I can't hold the gun steady so it's time to stop shooting !

    I have found the design of the 10.3g Jsb heavy much more stable ( less yawing) and far less pellet fussy batch to batch than the diabolo 8.4.
    I have used about 12 or so different batches of heavy and only found 1 that my gun did not group well with.
    I'd be lucky to find 1 in 6 with 8.4.

    The down side of 10.3 is your rangefinding past 45 yards needs to be with in 1 yard as they drop like a brick but more importantly you have to release the perfect shot each time.
    Any bad techniques amplifies the miss !

    The holy grail for me would be a
    8.5g " heavy !"
     
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  15. Ballisticboy

    Ballisticboy Active Member

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    Wind response is not a function of anything talked about in the OP.
     

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