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All Springer/NP/PCP Air Gun Discussion General > "Bob and Lloyds Workshop"

Light vs heavy pellet and long range accuracy potential

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Franklink:
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.

rsterne:
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....



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

It is possible we may be seeing a similar flattening of the drag curve when using a Meplat on round-nosed pellets....

Bob

Franklink:
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

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

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!

JungleShooter:

--- Quote from: Franklink on March 09, 2022, 07:15:41 PM ---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...."

--- End quote ---


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
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

Franklink:
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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