Ballistic coefficient change with range/velocity. The original study Feb 2010.

Forums General Discussion Ballistic coefficient change with range/velocity. The original study Feb 2010.

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    Yrrah
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    http://bsaog.co.uk/forum/index.php?topic=13228.msg98207#msg98207

    Please help.  Does this reference open for you to show my original report and data that has led to the subsequent use of variable and average bc input for long range pellet shooting.
    If it won’t open, could you please search Google using this search topic, “Promised 200 yards pellet Ballistic Coefficient experiment data and results. LONG. ”  And please let me know if it gives you access to the report and  data. Pictures won’t show because of Photobucket’s constraints, but I would like to put it back together for its historical value.

    It was done in Feb., 2010 and widely read at that time (7332 times) and played a part in the development of Dave Eades’  “GA base” within Chairgun.

    One of the associated graphs which you should be able to expand to allow the data to be read, but needs the above text to explain the details data  and rationale: I tried to post it in the pellets and projectiles section where it was relevant in another thread but having no response concluded it could not be read despite my efforts.
    Thank you for your help.  …  Kind regards, Harry G Fuller. Edit –  (If readable then any ensueing discussion is welcome).

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    Tominco
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    Hi Harry. 
    Yes, It brings me to the original post (without pics). I took a screen shot for you of what I see. 
    Your posts are always so very interesting and detailed. 

     

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    SMH77
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    Yes, it now opens for me.  The first time I tried it (the other day) it didn’t-for some reason, but now it does…

    Sean

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    SMH77
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    I’m in the process of conduction something similar, but rather over temperature changes, with a set (fixed) 1-50 yard BC test, with the same pellets, barrel, etc.  I have some of the data so far, and am waiting for temperature conditions to get more data (performing it in 20 deg F increments-roughly).

    I hope to compile a similar post with how the temp affects the BC’s of my test pellets-but obviously the work takes time (for temp conditions to change).

    Sean

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    SMH77
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    Great writeup Harry-It must be pretty hard to get the pellets into the correct window for the chrony at 200 yards!  

    After reading the article, and some of the ensuing discussion, I’m left wondering if the polygon barrels simply do a better job of spinning the pellet to stabilize it more quickly than the ‘rifled’ counterpart, for a given caliber?  Or does ‘cutting’ groove edges on the skirt (and head) of the pellet (rifled barrel) result in more drag than the more smoothly transistioned surfaces of (the same) pellets shot out of a polygon barrel?  See my chart below from the BC comparison thread, comparing measured BC’s from the .25 cal LW (rifled) vs. polygon barrels for the clear indication that the polygon barrel produces superior BC figures for the same pellets.

    One clue may lie in the fact that the Mk I heavy pellet (with a larger skirt diameter) seemed to be more aggressively affected by the different barrel condition compared to the ‘sized’ Mk II pellet.  Perhaps this is due to the ‘heavier’ cutting on that pellet (and thus pointing to the change in boundary layer flow conditions around the pellet’s profile)?  Similarly, the Mk I has a higher BC in the polygon barrel-is that due to more skirt material being deformed (since its a larger diameter) causing a slightly longer profile of the pellet (after it leaves the barrel), resulting in a better aerodynamic performance as it flies through the air (think of a ‘longer’ tail on a submarine, resulting in improved efficiency as it travels through the water, for example, versus ‘cutting off’ some of that longer ‘tail’ and suffering a loss of efficiency).

    From ‘part II’ of the other BC thread.

    Overall comparison of the barrel effects on the BC’s:

    I’d be curious to hear your thoughts on the topic as to what you suspect causes the clearly superior BC results in the polygon barrel vs. the LW (rifled) barrel.

    Sean

     

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    Yrrah
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    “SMH77”Great writeup Harry-It must be pretty hard to get the pellets into the correct window for the chrony at 200 yards!  

    Sean

     

    
 Even back then in 2010 good rifles could shoot the new Kings very well to 200 yd and further.
    When setting up for the above study and checking trajectory tables these three shots on an oil drum were made at 225 yards which was the limit to the sight setup I was using.
    If you expand the pic you see it is 2.16 inches and those are serious dents.

    Here is one of seven groups that averaged under three inches at 202 yards – shot consecutively.
    There are 4 shots in 0.55 inches  in the top of the small JSB tin’s circle used for a target. 

    So, given best conditions, if you have an accurate rifle and a shielded chronograph, give it a go. 
    Sean, I will endeavour to respond to your other questions as time permits. But  a quick reply:
    There is no question that a rise in air temperature makes air “less dense” and results in improved drag coefficient,   bc and higher POI .  You only have to go out on a clear frosty morning, shoot a group at 50 to 100 yards,  wait for the sun to come up and warm things up, shoot another group and so on to midday and you will see the effect. If you take the time to do bc tests coincidently, there it is. I have done it numerous times. The LabRadar just makes it easier.

    I will try to dig up a project that matched the BSA hammer forged barrel with the FX Elite ST  comparing bc and poi at 50 and 100 yards. Accuracy, bc and downrange velocities showed some very interesting results. Same pellets, same mv, same session match-up, but totally different shot pellet surfaces. Give me a little time, there’s a lot of stuff on file and not particularly well filed.  … Best regards to all readers,  Harry.

     

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    Yrrah
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    Thank you Tom.
    I think I have the way to do it now. Hope all is well in the high country these days.  Kind regards from OZ.
     

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