GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: Franklink on March 08, 2022, 08:55:47 PM
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Requesting thoughts on the following from minds who understand ballistics much better than myself.....
JSB .20/15.89 grain vs JSB .30/44.75grain pellet.
The .20 Heavy has a better BC than the .30, both in JSB's chart and in my measuring with the speed at two distances method. Mass is part of the BC formula but the 45 grain .30s seem to be a better long range pellet choice than the 16 grain .20s (less wind deflection from the .30s than the .20s). Why the apparent discrepancy?
If it was as simple as choose the better BC pellet we'd be seeing more .20s than .30s in the long range competitions....right?
If they're both going in the 900fps realm the .30 is going to have more energy left over @ 100 yards than the .20 started with at the muzzle. is there some component (inertia?) that makes the heavier pellet want to continue on it's path better than the .20, cuz I thought that was essentially what the BC is? Does BC not take account of the mass of the pellet to an appropriate degree?
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BC is to a large extent dependant on the SD, which is the "weight per area".... so yes the BC takes into account the mass.... The SD for the 15.89 gr. in .20 cal is 0.057, while for the 44.75 gr. in .30 cal is 0.071.... You would therefore expect the .30 cal to have about a 25% higher BC.... Since they are significantly different shapes, that likely accounts for the BC not tracking with the SD....
The problem with the BC is that if they are not measured at the same velocity, the BC changes, because we don't have a drag model that matches the pellet shape.... I haven't actually seen the BC on that .20 cal pellet, but if you have measured both, at the same velocity (and the one that you will use), then the pellet with the higher BC should perform better in wind drift, and for % retained velocity and energy....
Bob
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Chairgun has the BC of the 5mm JSB 15.89 as .030 for whatever that is worth.
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The background to this question was an Xtreme Field Target match we had here in AZ about two weeks ago. Large field targets out to 100 yards with a 100 fpe limit. So, fairly long range, at least for pellets.
Since inception of these monthly matches (second winter now), most of the competitors have gravitated towards the more mainstream (if that's a thing in airguns) long range pellets like the .22/25.4gr Monster RDs, the .25s, and the .30s. Those are coincidentally the pellets with better BCs. Most of these guys are pushing 45fpe and above. I personally have used the .22/MRDs every month except at the Feb match.
So at the most recent match I used the .20/15.89 and shot a 36/48. Now, that doesn't sound great compared to regular field target, but the winner shot a 39/48 (he was using .30/44.75gr) so I was pretty close to the top. And that winning 39/48 was one of the best knockdown percentages we've seen. I have shot the Heavy .20s quite a bit at home, out to 130 yards, and always been impressed with their performance, FOR SUCH A LIGHT PELLET. I have measured the BC of the .20s a couple different times and they average out to about 0.048. Which is quite high for such a light pellet, actually higher than most guys report getting with the .30/44.75 (including JSB's own BC chart).
The winner was able to hold within the kill zone with his .30 but I was needing to hold multiple inches outside of the kill zone to account for wind on the long range shots.
Bob, from your comments about the SD and from real world examples, the .30 SHOULD have a better BC. You think it is all due to shape? The .30/44.75 is a short squatty little dude while the .20/15.89 is more elongated and similar in shape to the .22/25.4gr MONSTER RD.
So this all creates some conundrums in my limited knowledge bank of external ballistics:
First, The .20/15.89 has an abnormally high bc, for it's light weight (which Bob ties back to the sectional density).
Second, the .30 is more (anecdotally) resistant to wind deflection than the .20/15.89 even though the .30 has a lower BC.
(as an aside, yes, I saw firsthand the effects of speed on a pellets BC when testing the .20/15.89. The BC is down around 0.038 when they're going 860 but up to 0.048 if they're pushed to 910-915).
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The BC is down around 0.038 when they're going 860 but up to 0.048 if they're pushed to 910-915
Right there is the key.... If the pellet shape matched the drag model you are using to calculate the BC, then the BC would not change.... A change from 0.038 to 0.048 with a velocity change of only 50 fps makes the data very suspect, IMO.... The pellet slows down as it goes downrange, so although you are launching it at 910-915, by the time it has passed 20 yards, it has slowed to less than 860 fps, so the BC is less than 0.038.... Without doing additional measurements at slower speeds, you really don't know what your pellet is doing.... With a BC(GA) of 0.048, starting at 915 fps it would be down to 685 fps at 100 yard.... Do you know what the BC is at 685 fps?....
The 44.8 gr. JSB .30 cal is one of the few pellets I have run through my LabRadar, and the BC(GA) is pretty consistent.... I used range increments of 10, 30, 50, 70 and 90 yds., and got 20 shot average velocities of 936 (muzzle), 903, 839, 785, 737 and 691 fps.... The BC(GA) as the pellet travelled downrange was very consistent, and calculated out to 0.041, 0.035, 0.035, 0.036 and 0.033.... The high value of 0.041 from 0-10 yards is probably due to the LabRadar underestimating the MV by extrapolating it from the data beyond 10 yards (where it acquires the pellet), while the pellet is slowing quicker during that time from its higher velocity, so I never look at the 0-10 yd. BC as relevant, and discard it.... Averaged out over 10-90 yards, it was 0.035.... The 90 yard data was getting a bit inconsistent (the SD was double what it was at 70, and one shot did not register), and the average BC(GA) from 10-70 yards was 0.036.... I have seen 0.037 published for that pellet in more than one place, so my results agree well with the claims.... This pellet seems quite a good match for the GA drag model, at least between 700-900 fps....
Wind is very seldom consistent in speed or direction over the 100 yards from muzzle to target.... If the BC of your .20 cal pellet is lower than the .30 cal once it drops below 800 fps, and the wind is the strongest closer to the target, then it would drift more.... BC is only a guide, as I have written before, what we really need is to be able to create a drag model for each pellet, from your own gun, and then use that for drift, trajectory and retained velocity calculations.... The only company I am aware of doing that is Lapua, but it is really the only way to create a model that does better than averaging too many variables....
Bob
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Thank you Bob, greater insight from your comments, as always.
I dug up the details on the BC collection I did, rather than just memory like I was working from before. Found this post I shared on a different forum, here's the relevant snippett...
"I first tried them with a muzzle velocity averaging 866 for about 26.5fpe (all 5 shot averages). They were still going 727fps at 54 yards so that gives them a BC of 0.039, which is….lackluster, and more in line with what I expected.
Next I tried them at a muzzle velocity averaging 908 for about 29fpe. They were still going 793fpe at 54 yards so a BC of 0.0495. Hey now! "That's the stuff!" I thought to myself, more in line with what JSB collected, and explains why these were hanging with the MRDs at paper in wind at 100ish yards a few weeks ago.
It looked like it was gonna rain on me at this point so I put everything away and had some family come over around then. After they left and the storm blew itself out I decided that I better verify that wasn't a fluke. Got everything out again and repeated the experiment. Got an average muzzle velocity of 912.7 and ended up with the far distance at 56 yards this time, and an average 56 yard speed of 784fps, working out to a BC of 0.0458, still promisingly high.
So, I averaged out the two for a BC of 0.04765 (0.048-just about what JSB reports) and plugged that into Strelok and plugged the clicks into the scope, and stretched it out a bit. "
I don't remember the ballistics program I plugged those speeds into, and now looking back, a 5 shot average isn't the most statistically robust sample.
I repeated all of that recently and the far distance was around 50 yards. BCs of the .20s @ 910 was in the 0.044 realm. Ten shot averages this time. Really dry day when the 0.044 was collected. Recently found out humidity actually lowers air density, which seems counterintuitive but makes sense with the previous testing between the rainstorms last summer. I'm attributing the better BC at that collection time to the humidity?
But, to answer your question, no, I've not tried to collect fps at 100 yards nor at figured out at what distance they're only going 685. Sounds like I need to do some more testing/experimenting.
Thanks again.
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Are you using your local air density and temperature to calculate the BC?.... Humidity is much less important, the change in density is very small.... but altitude or barometric pressure change the air density a lot, and temperature changes the speed of sound.... The Drag Coefficient (Cd) is dependant on the air density, and must be corrected to standard pressure and altitude before used to calculate the BC.... The velocity must be changed from fps to Mach number, before comparing the Cd to that of the drag model used to calculate the BC.... For pellets, the best current drag model is the "GA", but it's not perfect for all pellets (eg. useless for a wadcutter).... It is good for some JSBs, not for others, I have found.... You intuitively would think that longer, more cylindrical pellets should have less drag, but I have found the opposite is typical !.... Their increased weight, and therefore higher SD, helps of course.... but the Form Factor (FF) is often not as good as the more conventionally shaped JSBs.... Typical of this is the improved BC of the "Redesigned" Monsters, with the flared skirt and narrow waist, compared to the earlier model.... In most guns, they are also more accurate, although some still prefer the original design....
I have found some surprising things about pellet drag.... The ones I designed for NOE, with a 50% Meplat on the front of what is basically a JSB Exact shape, tend to have a better BC than the equivalent weight of JSB.... Here is what it looks like....
(https://hosting.photobucket.com/albums/oo221/rsterne/NOE Airgun Pellets/.highres/NOE 30 cal Hunter_zpscwrppghz.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/1ac04542-ec7b-4001-8236-2bd6c43b7532/p/8166e702-8a65-47a7-8167-7344ddc54167)
I tested the 45.7 gr. NOE Hunter in .30 cal the same day (same gun and settings) I did the 44.8 gr. JSBs, and got an average BC(GA) of 0.046 from 10-70 yds. (compared with 0.036 for the JSBs).... They were launching at 945 fps instead of 936 fps for the JSBs (despite being a grain heavier), and were arriving at 90 yards doing 738 fps instead of 691 fps.... Accuracy was almost identical.... The slightly heavier 48.4 gr. NOE Hunters (cast using shorter base pins) started out at 926 fps and at 90 yards were going even faster at 752 fps, for a average BC(GA) of 0.054 (from 10-70 yards).... arriving at 90 yds. with 27% more FPE than the JSBs, going 60 fps faster.... Their Cd was nearly constant over 750-900 fps, quite a surprise as well, as Cd usually increases about 25% in that velocity range!.... :o
Miles Morris has been developing a new Drag Model for airgun slugs, and has found that a moderate Meplat (25-50% of caliber) is actually reducing the drag in the Transonic range (800-1000 fps in particular)....
https://www.gatewaytoairguns.org/GTA/index.php?topic=187505.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=187505.0)
It is possible we may be seeing a similar flattening of the drag curve when using a Meplat on round-nosed pellets....
Bob
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I somewhat account for the density and temps but not to this extent..... "The Drag Coefficient (Cd) is dependant on the air density, and must be corrected to standard pressure and altitude before used to calculate the BC.... The velocity must be changed from fps to Mach number, before comparing the Cd to that of the drag model used to calculate the BC...."
I use Strelok and adjust the parameters for altitude and current temp, but let it just default to 29.53 inHg for the pressure. Enter the muzzle velocity and then I then tweak the input value for BC until the speeds @ x distance in the table match the 5 or 10 shot average speed the chronograph was picking up at that same distance. I've also plugged the two speeds and two distances into the BC widgets that some of the industry retailers have on their websites (AOA, PA, etc) and they produce similar BCs.
As for the longer, more cylindrical having less drag idea but the opposite often being true....I can agree with you 100% in that regard. I recently was playing with some of the .177/16.2gr and doing BC collections and long range performance assessments. I tried various speeds, up to 980fps, and the .177/16.2s don't do as well as the .20/15.89s (in BC and long range performance) even though the 16.2s are shaped like a long and tiny little dart compared to the .20/15.89 (which I'd call more similar to a diabolo). Both pellets weighing essentially 16grains, I felt it was a pretty interesting comparison. The .177/16.2s are very impressive for a .177 pellet @ distances up to 90 yards, but I'd choose the .20/15.89s or the .22/25.4 Monster RDs over them for such uses. BC of the JSB 16.2/,177 = 0.03938 (when starting with a muzzle velocity averaging 983.54). Collected with the fps at two distances method (muzzle and 52 yards). I'm not done testing the 16.2s though, they shoot so amazingly well, even in the wind, at distances out to 60 yards that I can't leave them be, and such a calm pellet to fire (Newton and all that).
More details here if interested: https://www.airgunnation.com/topic/veteran-long-22-rebarrel/page/2/#post-1194310 (https://www.airgunnation.com/topic/veteran-long-22-rebarrel/page/2/#post-1194310)
The meplat producing a better BC is also very intriguing. In that regard, I sized down some .22/18.13 grain pellets in 2 steps to get a 18.13grain .20. The sizing down process created a meplat and a BC of 0.042, better than I've ever measured with the 18.13s as a .22 with a round head.
More details here if interested: https://www.airgunnation.com/topic/20-jsb-exact-jumbo-heavies/?referrer=1 (https://www.airgunnation.com/topic/20-jsb-exact-jumbo-heavies/?referrer=1)
I also tried to size some of the .22/19.6gr NOEs down to .20 for a similar comparison but the process created such a long hollow skirt that the pellet probe in the test gun could not push them past the transfer port and with the air caving in the skirt when it got blasted, well, that tail of the experiment didn't get far.
You've rekindled my interest in the .22/18.13s as meplat'ed .20s now. I also need to do some research to see if I can figure out how to correct the Cd to current conditions, ala, "The Drag Coefficient (Cd) is dependent on the air density, and must be corrected to standard pressure and altitude before used to calculate the BC.... The velocity must be changed from fps to Mach number, before comparing the Cd to that of the drag model used to calculate the BC...."
As always, THANK YOU Mr. Sterne!
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I also need to do some research to see if I can figure out how to correct the Cd to current conditions, ala, "The Drag Coefficient (Cd) is dependent on the air density, and must be corrected to standard pressure and altitude before used to calculate the BC.... The velocity must be changed from fps to Mach number, before comparing the Cd to that of the drag model used to calculate the BC...."
Frank,
I somehow have missed a lot of your work with BC and ballistics -- maybe because the thread title featured a gun, not the word "BC", "ballistics", or a pellet name I was interested in....
It's great what you're doing, and I hope to get some BC testing on the way soon. I have collected the necessary chronos, lights, and finally a gun that has enough power to push those pellets I'm interested in.
You are talking about getting more precise BC calculations, esp. entering the atmospheric conditions:
🔶ChairGun for Windows (not the phone app) has wonderful graphs, and all the calculations one could imagine. The program allows you to enter atmospheric conditions, and then you can go over to BC calculations, which will use your atmospheric conditions for its calculations.
🔶 What I don't like about ChairGun is that I can't carry it in my pocket, meaning on my cell phone....
Now there is a new ballistic calculator, a full suite, with all you can imagine, and then some.
It is available for Windows, Android, Apple, and even for Linux.
Free.
As simple as these programs can get. Comes even with some instructions in each app.
GPC Ballistics Applications
Link:
https://gpc.fotosoft.co.uk/Home.html (https://gpc.fotosoft.co.uk/Home.html)
This app/ program also allows you to enter your atmospheric conditions.
🔶 A note to make our research data more useful to more people:
If we note how we calculated our BC numbers they will be more trustworthy, for example (and you have already given a lot of that info right here in this thread!):
method of measurement (2 velocities, or drop at range, or time to target), atmospheric conditions, drag model (GA, G1, GA2, etc.), ballistic calculator, MV, ranges at which you measured.
Cheers, keep up the good work!
Matthias
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Matthias,
Once you get some data of your own and start plugging it into whatever ballistic calculator you choose, you'll see for yourself that GA vs G1 and even Miles "new" GA2 all produce inconsequentially different BCs.
The BCs calculated by all of the ballistics apps for a given set of variables are functionally the same when using GA, G1, or GA2. At least when in the context of shooting pellets under 45fpe.
Yes, GA2 might give you a BC of 0.0413 and GA might give you a BC of 0.0421 and G1 might give you a BC of 0.0407 but in the realm of BCs, those are all essentially the same BC.
You've got to have larger differences in BCs before you actually start to "see" that in the behavior of the pellet in actual shooting conditions. (ie, a pellet/barrel/fps combo with a BC of 0.03 performs very poorly in windy conditions when compared to a pellet/barrel/fps combo with a BC of 0.04. BUT a BC of 0.0413 will have about the same wind drift as a BC of 0.0421).
People really get hung up on which drag law was used but they don't alter the predicted BCs much, at least for pellets under 45fpe (where I've done my testing and number crunching). Now, start throwing drastically better BC slugs like the 0.257 and the drag law might matter more, but it doesn't for pellets.
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Absolutely correct.... and calculating any BC to the third decimal is a waste of time, as a series of shots will vary more than that....
Bob