BC Variation of .25 caliber JSB pellets due to environmental condition changes

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SMH77

SMH77

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Oct 28, 2017
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Wisconsin, United States
Tonight I took some time to compile previously recorded BC testing data-all shot at the same range, under varying environmental conditions. All data shown below represents the same gun setup (RAW HM1000x in .25 caliber, shooting through a polygon barrel setup-with one exception noted below) with the hammer spring and regulator settings unchanged from session to session. Thus you can see the effects of temperature on 1 yard velocity, BC, etc. I did not do anything intentionally to alter the power output of the gun.

As a related topic, I did check variation with different speeds (adjusting hammer spring tension) today with my .30 cal setup as well, but that's a story for a different post.

You can also see the entry of the 'non-polygon' barrel below and how much lower the BC is out of that barrel than the rest of the readings through my polygon barrel in the same caliber, and on the same powerplant with no changes made in settings.

For reference, here's a chart showing the velocity change over temperature (at 1 yard) in .25 caliber to show how much it can vary:

1538889252_2939000095bb99624a90fe7.09543940_RAW .25 cal BC Variation with temps.jpg


Blue entry is the rifled barrel, all others are the polygon barrel:

1538890775_1993642065bb99c177cfca8.19983022_RAW .25 cal BC Variation with temps-cropped.jpg


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1538889263_6860326275bb9962f084745.01948176_RAW .25 cal BC Variation with temps.jpg


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1538890786_2278468585bb99c22b49303.31134605_RAW .25 cal BC Variation with temps-cropped.jpg




Hope this is useful information for those interested in seeing the variation effects. Please ask if you have any questions (or point out any typos).

I hope you find this info helpful? Is so, please consider taking a second and simply leaving a '+' with a nice comment for me-it let's me know my time and effort is appreciated and keeps me going with this kind of work for the community. I've spent a bunch of money on Chronographs now, lead and time so it's a pretty small way of saying 'thanks for the efforts'. Enough said- Enjoy!



Sean
 
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The BC is constant and is dictated by the shape of the pellet...

You just need to ALSO use the environmental conditions to make an accurate ballistic calculation of how the pellet flies through the changing air density.
Click to expand...

There's a lot going on there with your statement compared to the data and, I'm hoping, this will spurn a useful discussion that is constructive to the community. I wanted to make a few comments about what you've said-not from the stand point of 'I'm right' but to lay out there my thinking (currently) and make it open for correction, or correcting-whichever is appropriate based on whoever reads the comments...

First, on a fundamental level, I agree with the statement: BC is constant and is dictated by the shape of the pellet, however, I have a few points to make that affect oversimplying what is going on-and also to ponder from the data I've collected directly.

  1. The pellet, taken out of the tin, is in an initial state and has it's own 'as manufactured' BC associated with it's shape and mass distribution. Depending on whether the manufacturer has calculated this BC (theoretically or empirically) this data for the pellet may exist. If it was determined by the process of shooting the pellet, there's more going on (see points below).
  2. The 'as manufactured' BC is no longer present the instant is exits from the muzzle of the barrel as it has just completed going through a 'geometry altering operation' (going down the barrel of the gun, taking on it exterior the shapes formed from the inside of the barrel) that affects the pellets shape and (to a small amount) its mass distribution.
  3. The effects of point 2 above have a profound impact on the (what do I call it?) 'effective BC' (the BC, post exit from the muzzle, call it eBC) as it dramatically impacts Cd (Coefficient of Drag) of the pellet, and has an effect on how fast the pellet decelerates as it moves away from the muzzle. This is unique to which barrel the pellet was shot through and varies from barrel manufacturing approaches. In other words, the 'effective BC' on the same pellet (note again: the table I provided is from all the same pellet type-.25 cal JSB Heavy, 33.95 grain, Mk I-and each entry has been 'averaged out over a number of shots-in most cases 12 or more') a Lothar Walther 'traditionally rifled' barrel is different than the eBC of the same pellet shot through a Lothar Walther 'Polygonal' barrel, which is different than the eBC out of a FX Smooth Twist barrel, etc.
    1. For evidence to support this: look at the row in blue text above and compare the environmental conditions to other entries to see that it is dramatically different than what was produced at different environmental conditions through the polygonal barrel-despite being tested under similar environmental conditions.
    2. I also have data from my son's FX Wildcat Mk II, .25 caliber that show's his eBC is in a different range than those reported by my RAW. However, those muzzle velocities are different than mine (lower) and that too will have an impact on the trajectory due to different drags on the pellets at different speeds.
      [/LIST=1]
    3. The point above is why I still say that, for the shooter seeking to understand where to aim over different distances, it is still important for a shooter to measure the eBC from their pellet / barrel combination and use that to help inform their own ballistic calcs. I haven't yet studied the effects of barrel to barrel variation (using the same 'type' of barrel-for instance: two RAW's both shooting .25 cal mk I heavies through polygon barrels)-but I will be able to once I can get access to my buddy's gun. This won't be an extensive test, but should give a small sample of what kind of variation is present between barrels of the same manufacturing process.
    4. While the pellet shape is 'set' upon exiting the muzzle, the rate of deceleration, setting the drop curve, varies over distance, as a result of the lower velocity at different distances resulting in a changing drag force on the pellet. I don't have data to back this up (yet) but I have read similar observations and have seen it in a limited fashion when testing the eBC from muzzle to 30 yards, and 1 yard to 50 yards, etc. Others like Harry (Yrrah) have also noted the same thing in posts they've made. The trend is that the eBC gets higher across a longer distance of comparison. This causes it to 'act like' a changing eBC over the course of its flight. I don't want to make too grand of conclusions here until I have data under the same environmental conditions showing the eBC as measured at 10, 25, 30, 50, 75, and 100 yards (etc) and can plot the changes over distance from the first reading (at 1 yard). I believe Harry has already done, and posted, some of this data on nthe Yellow Forum in the past-I plan to duplicate it myself so I have my own data, for my own barrel / pellet combo.
      [/LIST=1]

      Second, with the context of the points laid out above, I would like to make a few comments about the second point you made (just need to ALSO use the environmental conditions to make an accurate ballistic calculation ...)-which I also agree with-as an explanation for why I'm testing and posting the information here for the community.
      1. There isn't a lot of information (that I know of) that helps a shooter to 'adjust' a BC for changes in temperature, pressure, humidity, etc. It is generally known that these factors influence the trajectory of the projectile, but I haven't seen a calculator to actually predict the change in the eBC based on a conversion of environmental conditions.
      2. That brings me to 'why' I'm posting my data for others to see/study/refute/learn from: this is a practical, real world example of how the trajectory results can vary based on changing environmental conditions. I know that my gun's POI at varying distances is notably different when I'm shooting in the fall, winter or summer-and the data documents that and gives a reader an example to see that in action (hopefully to help them grow their own understanding of what they've seen also). Once understood, it can help a shooter make the needed changes to aim point / clicks / etc to compensate-or just help them understand that they are NOT going nuts, and that what they are experiencing is very real indeed.
        [/LIST=1]

        Anyway I probably have more points to make about this as well, but I'll leave it at that for the moment. Suffice it to say that I essentially agree with your statements-but there's a lot more going on that I'm trying to extract to help others out in the process. Hopefully it will help us all learn/grow our understanding together.



        Sean
 
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I see what you are trying to attempt to do which is to "adjust" the BC to conditions when the total solution of a fully real-time atmospheric-aware ballistic calculator isn't available. Your approach is valid as such.

I just wanted to be clear in the physics of a particular shape/mass/Cg object flying through a medium.

And, I agree the shape that counts is the one that exits the barrel.



No argument intended, just a little definitional clarity.


 
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Agreed. Most shooters 'know' that the temp/pressure/humidity play a factor but, without an example to see how dramatic (or not) the effects are, they are left guessing as to how much to allow these factors to make an adjustment to their shooting (if they should at all). I plan to create some plots of eBC vs temp, eBC vs. pressure, and eBC vs. humidity to show whether it varies much with thos factors and to give an indication of how much.

There are other 'consequential' learnings to posting the data too: I've seen threads where people argue as to whether (and how much) muzzle velocity gets affected by changes in temperature, for example. My data above demonstrates that pretty well too (by comparing the '1 yard' velocites vs. temperature-which I could plot too). Obviously at 1 yard, the main factor (keeping the barrel constant) is changes in temperature as the pellet doesn't travel far enough by then to be significantly impacted by much other than temperature-or perhaps I'm oversimplying that myself? I guess it would be 'energy present in the compressed air' and 'changes in the frictional coefficient with temperature' that would factor in for that variation-again, lumped together in the data it at least puts a number to what people have argued about in the past.

One other comment about the 1 yard velocity when looking at the results from the polygonal barrel vs. the traditionally rifled barrel: it's interesting to see how much less friction the polygonal barrel exhibits on the pellet compared to the traditionallly rifle LW barrel, as can be seen from the differences in 1 yard velocity. This accounts for a ~24 fps loss in friction right out of the barrel-that's pretty dramatic...



Sean
 
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This is precisely why I programmed my RW to shoot 930, 940, 950fps.

I'll be visiting AZ next week for EBR. My rifle is zero'd with an Altitude Density of 3370 at 87 degrees F. The temp in AZ is usually 10 degrees higher, but it looks like this year it'll be 10-20 degrees lower.

So I've got a few choices:

A. I can take a new Altitude Density reading in AZ & use the new data it spits out for new holdovers.

B. Change my power setting to accommodate for the temperature difference so I'd be shooting roughly the same velocity as I a here.

C. A combination of A & B

At least that's my plan 😉

Thanks for sharing your findings. I'll share more after EBR.
 
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Another thing I'm finding very interesting is how the BC changes at difference velocities. For example the BC of my 25g .22 caliber pellets starts off at .059 & decreases significantly after 80 yards.

I could use a single BC of .046 (v20 - v100) but I'm finding it more accurate when I enter multiple BC's.
 
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BC is an attempt to reduce the drag function to a number - a crude approximation.

As such, it is valid to use different BC's at different velocities.



What you really want is drag-vs-velocity data which is why devices like LabRadar have the potential to yield more accurate ballistic predictions when coupled with the right calculator that can also map sight-in time atmospheric conditions to current shooting conditions.
 
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BC is an attempt to reduce the drag function to a number - a crude approximation.

As such, it is valid to use different BC's at different velocities.



What you really want is drag-vs-velocity data which is why devices like LabRadar have the potential to yield more accurate ballistic predictions when coupled with the right calculator that can also map sight-in time atmospheric conditions to current shooting conditions.
Click to expand...

Love me some Labradar!

1538946330_8042189685bba751a11cf95.08723290_20181003_214010.jpg


1538946312_6959295235bba75085d8122.85773954_Screen Shot 2018-10-07 at 4.04.32 PM.png

 
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Keymaster - does the LabRadar work well? For smaller calibers like 0.177 and 0.22? Have you ever used it indoors?
Click to expand...

Hi Fred, the names Michael

Yes the data I shared above is from a .22. And others, like my YouTube partner Steve at AEAC, have successfully used it to track numerous. 177 pellets.

It'll give you a breakout of the velocity in 5 yard increments out to at least 100y (when you use a SD card to store the data). 
 
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Simply put BC = (Weight/700)/(caliber * caliber* form factor)

Since form factor is the only variable and is calculated by the drag coefficient of an ideal pellet divided by the drag coefficient of the actual pellet and the drag coefficient is based on velocity then:

BC is based on velocity.

As for temperature, humidity, barometric pressure you normalize to ICAO standards and then can calculate for new temperature etc from that base.
 
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I am going to try and answer this a different way. I will combine your two posts into my answer.

Your second post asked for discussion and correction. I am going to answer in that spirit. If it gets heated I will stop. Consider "you" as a generic. I am not here directing this at the real you. I am just discussing what was posted. I fully understand you are a human behind your keyboard and as such you have value.

In your first post you see a pattern. Possibly you do. I don't. I am not saying there is not a pattern. I am saying you have not broken the data down enough mathematically to prove a pattern. I will cover that toward the end because we need to get on the same page in your second post first.

I will also state up front that the drag coefficient is the only thing that matters in this discussion. I am not going to prove this simply because if you look at the math it is easy to prove on the deterministic elements and if you look at non-deterministic elements (wind, etc) it still all falls right back to drag coefficient.

I am going simplify a few things because we are staying subsonic.

You state the BC is constant. That is incorrect. Your data in your first chart is the summing and averaging of all BC's that pellet went through from the muzzle to 50 yards. Here is how I now this. If you took the fps at 49 yards and at 50 yards your BC is lower than if you took your fps at 1 yard and at 2 yards. Your BC would also be higher if you took your FPS at 1 yard and at 1 1/2 yards as compared to 1 to 2 yards or 1 1/2 yards to 2 yards. What you have done in a crude way is a differential equation. Nothing wrong with your technique I just want you to understand that every millisecond, or microsecond, or whatever measurement you gather data by the BC is not only different it is lower because the velocity at farther distances is lower. If you don't know what a differential equation is then it is a multi-variable calculus formula.

You also talk about the BC changing as the pellet changes form from being fired. Fair enough. That is called internal ballistics. There is a whole science around it. Once it leaves the barrel it is external ballistics. External is generally all you can control without being a gunsmith. However, you have stated that a polygon barrel does less deforming than a rifled barrel. Does it? I have no idea other than intuition just like you are using intuition. You have not proven it. So, your theory is as good as mine and neither maybe correct. So, I won't state mine. (Points 2 through 4).

Point 5 is exactly right. Drag coefficient varies by fps which is exactly why BC varies per fps. So paragraph 2 about BC being constant conflicts with point 5. If you want to see it empirically look at what is printed on match grade long range boat tail slugs from name brand manufacturers. They have used Doppler radar to capture BC over different fps and give you the BC for different ranges of fps. There is one correction: I believe you meant to say "eBC" gets lower over distance, not higher as you stated. It has to because slower velocity of the same cross sectional density (e.g. same pellet) always gives lower BC (generalized).

Point 1 at the end of the second post. Yes it affects "trajectory" which is a term I really dislike. But that is only because it effects the only thing that is important: the drag coefficient.

Finally I am going to discuss how you pull meaningful information out of the first post. To do that I am going to cover simple stuff and am going to wave my hands around the math.

Hot air has less drag than cold air because it is thinner.

Higher humidity air contains more water and if you have ever been in a pool you know it is harder to run in water than air. So, higher is more drag.

Barometric pressure is how many atmospheres are pushing down. The more down force the thicker the air and the more drag.

One more point. Every thing we have discussed has been external ballistics and deterministic factors. We have not even discussed things like wind which are non-deterministic in nature. So, as you progress don't forget those. As an example if the wind is coming from your left the head of the pellet turns into the wind (to the left) and the tail or skirt moves the opposite (to the right). You may have a stable pellet that is still moving in a straight line but is canted sideways. That is obviously less aerodynamic and increases drag.

Let me tell you what you can do with your data. You can fire multiple pellets on the same day with the same temperature, humidity, and barometer and average them together. You can then pick a standard. I used the ICAO standard and through math adjust (correct) your temperature, humidity, and barometric pressure to this standard which also changes your calculated muzzle fps and 50 yard. So, your BC would also change. Then theoretically you don't ever need to even have any of the other polygon tests. You just vary the ICAO numbers to match your real world calculations for the next day, or the match, and it will tell you your initial fps, 50 yard fps, BC, drag coefficient, etc. That is where the pattern is.

Now, the rifled barrel. Simply put we just normalized your data for the polygon barrel in the paragraph above. Fire the rifled or x-twist, or any barrel in any gun on the same day, average the results, normalize the data to a standard, and look at the BC. Ah, but there is one issue. Simply put you can't speed a pellet up in the math. You can only slow it down. i will give an example. If you fire the polygon barrel and the rifled barrel on the same day but the fps for the polygon is 1000 and the other is 900 then do everything to calculate the BC on both pellets you have to throw all your data away on the polygon barrel from 1000 to 900 fps. Both pellets have to be at the same starting fps to mean anything.

Look at most of the BC data out there in the air gun world. It tells you nothing. If you (made up) fired an H&N 15.3 grain pellet at 980 fps and a JSB 15.3 grain pellet at 1005 fps generally the form factor and thus the BC will be higher for JSB. That is exactly what you see in the BC data. Reverse the fps and the H&N wins.I will admit this is not always the case and I actually think that is where your point on internal ballistics rings true. Bend a skirt edge 90 degrees and no matter what you do drag coefficient goes wonky. the only way to prove this is to make some really goppy ballistic jell, hope it doesn't deform the pellet and visually inspect it.

My opinion is (and you know opinions) that it is because the cost of the pellet is in pennies and in the seriously competitive end of the sport it is easier to match the pellet to the gun/barrel and then buy a boatload of pellets with the same lot number than it is to do the math. That is just a theory with no proven facts to base it on at all (conjecture).

Finally, I don't use ballistic programs. I have two reasons, but will only discuss one. I will never shoot a pellet over 25 meters. The target I shoot will always be at a fixed distance. Doesn't matter what it is: 10, 11, 15 meters, etc. it will never move. I envy you guys who can. However, if you do be very careful in what standardized ballistic coefficient you use. Without naming names: the pinned post on AGN uses a standard G1 coefficient. I believe some software packages utilize GA. If you calculate your BC on the G1 standard and then don't convert it to the GA, G7, GL or whatever standard your ballistics software uses your final results will be wrong.
 
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Good reply-there's a lot in there to unpack. I'm going to digest it a bit and respond to bits and pieces-and don't want to make this an argument at all as I would like this to be a constructive, informative thread for myself and others...

I do have two quick points though (1 error, 1 comment/clarification).

This statement has one of them flipped (see below). Look it up and you'll see I'm correct. I've also witnessed it first hand with cycling by studying power meter data vs. speed. Not much to debate on this one. I can also point to page 169 in Ryan Cleckner's book "Long Range Shooting Handbook" where he says that humidity is frequently mis-understood and people think that higher humidity = more drag, when in fact the opposite is actually true (its counter-intuitive)...

"Hot air has less drag than cold air because it is thinner. -Correct

Higher humidity air contains more water and if you have ever been in a pool you know it is harder to run in water than air. So, higher is more drag. -Incorrect

Barometric pressure is how many atmospheres are pushing down. The more down force the thicker the air and the more drag. -Correct"

The other comment-you mis-quoted me and I did not say the following (go back and read my un-edited post...):

"However, you have stated that a polygon barrel does less deforming than a rifled barrel. Does it? I have no idea other than intuition just like you are using intuition. You have not proven it. So, your theory is as good as mine and neither maybe correct. So, I won't state mine. (Points 2 through 4)."

Not to start an argument, but what I said was this:

"One other comment about the 1 yard velocity when looking at the results from the polygonal barrel vs. the traditionally rifled barrel: it's interesting to see how much less friction the polygonal barrel exhibits on the pellet compared to the traditionallly rifle LW barrel, as can be seen from the differences in 1 yard velocity. This accounts for a ~24 fps loss in friction right out of the barrel-that's pretty dramatic…"

  • I did not say the pellet is deformed more by the traditionally rifled barrel-just that the barrel exhibits more friction on the pellet. 
  • The evidence I point to is the following:
    • the powerplant remained unchanged (no changes in hammer spring tension, or regulator pressure)
    • the pellets were from the same tin (no changes)
    • the gun was filled from the same scuba tank (no changes in humidity or composition within the power source)
    • the temperature during the tests were different, but that was corrected in the comparison data (see my post for details on the correction)
    • the temp effects on the muzzle velocity equate to ~4.7 fps difference (based on the best fit equation to the data) between temps (and at 1 yard, differences due to pressure and humidity are neglible)

I'm merely pointing out that I'm using more than just raw intuition-but rather data-to support my comment about the difference in BC between the two barrels.

Regardless-and to prevent an argument based on data that can be argued-I'll offer this: in the next couple days (hopefully when the conditions are fairly calm), I'll shoot both combinations, back to back, in the same environmental conditions. It takes about 5 minutes to swap barrels so it's no big deal to just squash the debate with hard data. I will use the same chronographs, setup, pellets, etc. to document. If I'm wrong, no problem-I'll gladly point that out.

Like I said though: you brought up several good point to chew on-and I will need to think/study them some more.

One last comment: I guess I missed the fact that the 'form factor' is more than a geometric ratio, but rather includes the aerodynamic drag function as a consequence. I guess I was equating the BC to the purely geometric packing of material into a volume (which, admittedly caused me some confusion as to why it would change then), and I was reserving the changing rate of velocity loss (deceleration) to the drag function. Thanks for pointing out the error and clarifying that for me. It's causing me to dig deeper and understand it better.

I hope this reply is read with the correct tone as I intended it to carry...



Sean


 
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Sean,

Great reply.

I will go re-look at humidity.

Fair enough on my mis-interpretation on friction vs. deformity. I guess I am struggling with deformity as a factor, and have discussed it with someone else about what percentage of BC this really effects if you performed all of your BC raw data from a single barrel. I will freely admit I incorporated that into my reply. No need to go back and re-due your tests to demonstrate you are correct. Although in all honesty, it sure would help me answer the problem running around in my head ;)

Form factor is where I believe the holy grail is. The formula is one of an ideal drag "curve" divided by an actual drag value. I use the term curve loosely, and unscientifically. But if you look at G1, G7, etc they are used to take what you discovered empirically (drag value) and modify it to other values on the drag "curve". Through reading the posts on here over time the ballistic software developers have this close enough that people are getting good results on theoretical scope vs real world scope POA.

Your last comment was really why I replied. I was long winded, but I really just wanted you to understand the point you stated more eloquently since it is a critical piece in where you are going.
 
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