Is Ballistic Coefficient a Constant?

Forums Pellets, Projectiles, Slugs, & Ammo Is Ballistic Coefficient a Constant?

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    mmahoney
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    I just read another post and am now confused and need a good explanation. Is ballistic coefficient of a particular pellet a fixed constant?

    My current (possibly wrong) thinking: The shape and weight of the pellet determines the pellet's ballistic coefficient. This value is a set constant that I imagine as being the slipperiness of the pellet as it travels through the air.  This slipperiness does not change unless something changes the shape or mass of the pellet such as rifling grooves from the barrel. Variation in the way the pellet flies in the wind is a result of air temperature, humidity, pressure, wind velocity, etc. but these do not affect the pellet's ballistic coefficient. Point of impact can be affected by the temperature, humidity, thickness of air, angle of shot, etc. but these are different variables in the overall equation given by Stelok or Chairgun. Once I know the ballistic coefficient of the pellet through my barrel (probably real close to that reported by Hard Air Magazine's data table) I don't need to worry about the BC of my pellets ever being different.

    Am I right or where am I wrong?

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    18.13
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    http://www.pyramydair.com/blog/2014/05/ballistic-coefficient-what-is-it-part-1/

    Here is a little bit of info    short answer no but please don't make me explan

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    stoti
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    The BC of a pellet can and does change, most notably with velocity changes. For our uses though, once you find a velocity that you'll have your gun tuned to and the BC at that velocity, you can use those numbers to punch into your ballistic calculator and you'll be fine. Like 18.13 said, don't make me explain. It's obviously a lot more complex than that but sometimes, in most cases actually, this will be good enough for air gun ranges.

    If you want to read more on anything associated with ballistics, do a search for Brian Litz. He's the ballistician for Berger bullets and the man who started Applied Ballistics. He's probably the foremost authority on these types of things. He has 3-4 books out on the subject that are packed with useful information. Not the most fun reading, but if you want to really learn about this stuff, mandatory reading.

    Stoti

    • This reply was modified 5 months ago by stoti.
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    mmahoney
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    Thanks for that link, 18.13. I copied this from the link you shared.

    So if the BC of a projectile varies because of different amounts of drag at different velocities, which number gets reported as "the" ballisitic coefficient? The max? The mean?

    I know there are different models that are used to approximate this affect of drag at different velocities….GA, GU, G1 etc., but always thought that the ballistic coefficient was the coefficient and the drag was a different variable.

    For example, when shooting at 5000 ft my pellet experiences less drag than at sea level. If I shot the pellet underwater, it would have significanlty more drag. A good trajectory calculation would need to include the fact that the environment is different but not that the pellet is. Am I wrong when I assume that the drag law (GA for example) is supposed to take into account the environmental differences instead of the ballistic coefficient?

    I guess I'm still trying to get my head around why the darn number isn't called the ballisitic variable instead of the ballistic coefficient.

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    PerkyVal
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    After I bought my Lab Radar I read a bit about it as it would give you accurate fps in increments.

    I've since sold the unit, but as I recall the speed of the pellet, as well as the usual environmental conditions such as elevation, barometric pressure, temperatures, etc affect BC.

    My conclusion was that it all hurt my head too much to bother with, and as Stoti insinuated, at the distances, FPS, FPE, regulator inconsistencies etc a .003 discrepancy in BC really isnt worth worrying about, with how, and what we shoot other than as a mental exercise. Unless you're writing a balastic program, or really really need to satisfy curiosity.

    I typically use chairguns BC, or Hard Air mags.

    There was someone.that posted here sometime ago that went thru the calculations with a Lab Radar for some common pellets. I dont recall any of those results to be significantly out of line with what we commonly use.

     

    Have fun storming the castle!

     

     

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    fishinwrench
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    Well then…. since the velocity changes drastically from muzzle to target, this whole BC thing seems like junk science.

    We are never zeroed to within .150" on every shot anyway, so there is a point where close enough is close enough.   For me it's all about killing critters and spinning spinners, not lighting matches at 50 yards. 

    😊

    I say:.  Just assign a BC to each pellet that makes the ballistic calculations work as close as reasonably possible, and skip the OCD behavior.

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    Michigander
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    I hesitate to respond to this thread because it is a rabbit hole. Chasing the BC that predicts actual results within 0.1" at all ranges with your choice of gun and pellet is not likely to be successful. The charts, tables and calculators should get you close, but you should expect to have to do some experimentation and fine tuning to perfect your turret settings. Let me try to explain why this is.

    First, it is important to understand that BC describes the behavior of your projectile with respect to a reference or model projectile. If the BC is 1.0, it will have the same drag characteristics as that model projectile. But even for that model projectile, drag characteristics, specifically the drag coefficient, will change with velocity. If your projectile's drag characteristics change with velocity in exactly the same way, then one BC will apply at all velocities. But this seldom happens, which is why Sierra publishes separate BC values for each velocity range. If the above chart from pyramid air is correct it means that the way in which the described pellet's drag characteristics change with velocity is not the same as for the model projectile (G1?).

    But if someone were to come up with a projectile model that very closely described the way a real pellet's drag characteristics change with velocity, then one BC could apply at all velocities. And I have heard of someone attempting to do just that, although I don't know how successful the effort has been. Some of you probably know what I am talking about. It should prove to be an improvement on simply trying to use the standard G1 model for pellets.

    Now I will mention one other fact.  Projectile form factor can, and often does, change with distance. A bullet/projectile will leave the muzzle with some degree of yaw, giving more drag than if it were perfectly aligned with the direction of travel. A well stabilized bullet or pellet will turn to realign itself (go to sleep), poorly stabilized bullets or pellets may develop more yaw downrange, giving those spiral paths you see in videos in extreme cases. A ballistic model will have to assume either perfect stability, or one of these other two scenarios. Which scenario applies to your situation? It is things like this that make it highly unlikely that a ballistic model is going to fit your situation exactly and why some fine tuning should be expected.

    I hope this has been helpful. The last thing I want to do is add to the confusion.

    Chuck

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    mtnGhost
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    @michigander, well put. I think BC is misunderstood (and misrepresented), the commonality that I have seen in the airgunning community is using it as a reference value between ~muzzle and 50/100 yards as a quick gauge of a projectile. 

    I calculate the BC for the ammo that I shoot long range using my LabRadar data, where I use the BC numbers as a reference value of a projectile's ballistic characteristic between multiple sets of reference point pairs. I do this mainly for gauging how the ammo performs between A and B (where A is not always at the muzzle) – and I like to do this comparatively for a given velocity tune in different wind conditions, e.g. comparing the data on a calm day against a day with a lot of crosswind … and a day with tailwind; you get my point :)

    I also like to generate graphs for projectile flight (mainly G1 since I shoot "slugs") when I'm shooting at specific velocities to see the drop in inches, and for visualizing the general expectations for drift. I will often record my shots with video and calibrate them with my LabRadar shot series for later reference – especially when there are anomalies that invalidate my drag coefficient plots (for better or worse). Subsonic small bore is a very interesting arena, almost nothing about it is linear!

    One observation with the  LabRadar – it's fun, useful, and convenient (compared to shooting through Chronys!), but I've found environmental factors can throw it off at longer distances — more so with the smaller bore ammo like .22 cal. I haven't had those issues with .30, or even the times that I shot some .25. It can be frustrating! 

    I realize that everything I'm doing is all very primitive, there's just so much real-world variability in play and the tools aren't perfect. I work towards finding the optimal long range tunes for my rifle + ammo combinations  – finding the max velocities that I can use before the undeniable force of drag has its way to disrupt things. Studying the data gives me ability to make better tuning decisions for given wind conditions and for setting my targets out to certain distances. That allows me to challenge myself in a way that I do not get bored, while I'm out shooting for a solid 8 hours or so (on the infrequent occasions that I drive 150 miles round-trip to go out and shoot!) 🙃 

     

     

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    mtnGhost
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    One thing to noodle is a great quote from one of Bryan Litz's books where he said:

    The biggest challenge of using ballistic coefficients effectively is managing the velocity dependence.

    (Applies Ballistics for Long Range Shooting)

    • This reply was modified 4 months ago by mtnGhost.
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    Haganaga
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    Doesn’t barometric pressure and temperature effect it as well?

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    mtnGhost
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    Haganaga

    Doesn’t barometric pressure and temperature effect it as well?

    Yep .. barometric pressure / altitude / temperatures.

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    mmahoney
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    All I can say is that it is no wonder manufacturers don't list the BC on their pellet tins. The BC can vary so much that it is pretty much pointless without a standard for BC measuring conditions.

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