Ballistics Coefficients Discussion

[Tomg]

Cool, I'll do some testing tomorrow, or Friday from 1yd to 25yd.

[rsterne]

If you have the ability to test at different velocities, yet with a stable ES, we need data from 1000 fps and slower, down to 500 fps or less.... At the moment I am concentrating on the JSB Exact series, but all data is welcome, although may not influence or be included in the eventual drag model we develop....

Bob

[rsterne]

I received more data from Michael today, and added it to the database.... Here is a detailed view of the high subsonic and transonic region....



The lines look a little wiggly, but when you consider they were take by three people, in three locations, with four different barrels and everyone using different batches of pellets, I think it is remarkably consistent.... I still want to fill in the data between 900-1000 fps, and below 750 fps, of course.... Note how close the data for the AA and JSB pellets sits on top of each other, 4 of the 6 AA data points lie right on the JSB curve....

Bob

[Tomg]

Educate us ohh, Bob the grey.

Joke aside, this show how much drag they have, so lower number is better.
I think the JSB 15.9 grains shows a lot of promise....somewhat better than the 18grains in the 800-880 area.

[vinenator]

Wow, a LOT of reading to catch up on.

Not sure if it was touched on since I haven't had time to really read all the responses, but it seems to me this problem (is BC a constant at various velocities) reminds me of a similar type of question in astronomy (is the expansion of the universe a constant).

if, with BC, if you assume it isn't, you want to figure out how measurements differ from the "flat" line of being constant. The velocity tests seem to be a logical way to do this since if the BC is flat, a very calculable outcome can be estimated. But getting good V1 with relationship to V0 seems to me to be the hurdle.

we are dealing with some very small variations, at least subsonic and that is fine for me since I don't plan to be shooting anything over M1 anytime soon. Getting rid of as many variables as possible is going to make data reduction a whole lot easier. I see comments about measuring muzzle velocity and down range velocity. I didn't notice, but are two chroneys being used for these tests so the V0 and V1 measurements are related to the same shot? the muzzle velocity, even out of a regulated rifle is going to change enough to throw off the dV of any test pair. So, two chroneys definitely need to be used. Then you need to normalize Temp, Humidity, and stray winds. This definitely sounds like an indoor test but trying to do measurements past 20 yards is going to be tough unless there is a 50yd firing range available.

If we could create an SOP for doing the test, then a whole lot of people could participate. but like the scientific method describes, the tests have to be repeatable. Even accuracy of chronographs need to be taken into consideration to insure the error bar isn't so big that it makes the data worthless.

I think with all the engineers out there, some data collecting methodology can be described.

do the pros publish their test methods?

[rsterne]

Two Chronys have the problem of making sure they are identical.... Using the average of 5 shots is not a perfect solution with using a single Chrony and moving the location, but for most of us, it is really the only solution available.... While I would agree that the data we are collecting is not perfect, it is a LOT better than what is currently available, which is base on the form of a spitzer bullet, or a simple assumption that drag is a constant, which we know is NOT the case.... Given enough data, I think we can come up with a model that may approximate the best pellets that are currently available, and that it will be a large improvement over what we have now, which drastically underestimates the drag increase in the transonic region....

Bob

[Setarip]

Hey Bob,
 
This upcoming week I should be able to obtain some data on 18.3 jsb's below 750 fps for you.  I currently  can push them over 800 fps, but my gun being a pumper I can really set the velocity to whatever you need below that. I will try to get data points for 500, 600, and 700 fps when possible for you.

[Tomg]

I agree with Bob, empirical data, granted with some level of error/variation, still proves trends that can provide a good basis for calculating a better BC for airgun pellets.
On that topic it baffles me, that the large pellet manufacturers like JSB doesn't do this themselves and release the data chart to aid in user setup.
It seems that would provide data to them as well, maybe also to the point of making better designed pellets, since such is a result of iterating based on data collected.

[rsterne]

The Drag Coefficient of the G1 Model is roughly 3 times as great at Mach 1 as it is at Mach 0.5, but in fact pellets have 4 to 5 times as much drag at Mach 1.... This really emphacizes how little sense it makes to try and push supersonic, and how much more wind drift you will get in the transonic range.... Even if all we accomplish is to establish a model that has a much larger increase in drag in the transonic region, it will allow a much better understanding of the futility of driving pellets supersonic or nearly so....

Bob

[sixshootertexan]

JSB .25 25.4g 68° 30.25in Humidity 68% 233' elevation

1yd          26yd
755          686
753          688
758          686
755          689
755          689
755          690
755          692
758          688
751          690
754          688


1yd         26yd
722         659
725         660
723         662
725         659
721         663
721         661
717         666
721         659
720         662
719         664

[rsterne]

Thank you for the addition of that data on the .25 cal Kings.... It falls very close to the data for the .22 cal .18 gr. Heavies at the same velocities.... I used the averages of each 10-shot string....

Bob

[Setarip]

Hey Bob,
Since we know that pellets experience 4-5 times more drag when supersonic, I thought of a couple of points. Let's assume we fire two jsb pellets, one at 1200 fps and the other at 850 fps for this thought exercise. Since the supersonic pellet is undergoing a significant increase in drag, will the time of flight difference between the two pellets be much different at 50, 75, or 100 yards? Which pellet will carry more energy to the target at those distances?

[rsterne]

The one that starts faster will always have a shorter time of flight and greater retained velocity and energy at a given distance.... However, the slower one will have the greatest PERCENT retained velocity and energy.... The faster one will have more wind drift at all distances....

Bob

[Setarip]

Very fascinating. What's interesting is th greater wind drift with the faster projectile. It makes sense since the BC is lower, but at the same time it arrives at the target faster so it would seem that it is exposed to wind gusts less and should therefore move less. Weird.

[rsterne]

You need to read up on wind drift.... The flight time is not the governing factor, it is the DIFFERENCE in flight time between the real world and what would happen in a vacuum.... The BC, or more properly the Drag, is the governing factor....

Bob

[Setarip]

Interesting. I am just getting into the physics of all of this stuff so I probably should! Lol.

[Setarip]

So I just did some calculations on the ballistics of a jsb with .043 BC at 819 fps, and one at 1100 fps with an assumed BC of .015. This may be over simplified, but interesting:

Jsb 0.043 at 819 fps:

Muzzle fpe: 27 fpe

25 yards: 758, 23 fpe

50 yards: 704, 20 fpe

75 yards: 655, 17 fpe

100 yards: 609, 15 fpe

Average velocity: 714 fps

Time of flight: .42 seconds




Jsb 0.015 at 1100 fps:

Muzzle fpe: 49 fpe

25 yards: 813, 27 fpe

50 yards: 657, 17 fpe

75 yards: 533, 11 fpe

100 yards: 430, 7 fpe

Average velocity: 765 fps

Time of flight: .392 seconds

The energy bleeds off very rapidly at 1100 fps and really the .03 secs faster flight time has no major advantage and is carrying much less energy (assuming my calculations are correct).

[Scotchmo]

So I just did some calculations on the ballistics of a jsb with .043 BC at 819 fps, and one at 1100 fps with an assumed BC of .015. This may be over simplified, but interesting:

Jsb 0.043 at 819 fps:
Muzzle fpe: 27 fpe
25 yards: 758, 23 fpe
50 yards: 704, 20 fpe
75 yards: 655, 17 fpe
100 yards: 609, 15 fpe

Average velocity: 714 fps

Time of flight: .42 seconds


Jsb 0.015 at 1100 fps:
Muzzle fpe: 49 fpe
25 yards: 813, 27 fpe
50 yards: 657, 17 fpe
75 yards: 533, 11 fpe
100 yards: 430, 7 fpe

Average velocity: 765 fps

Time of flight: .392 seconds

The energy bleeds off very rapidly at 1100 fps and really the .03 secs faster flight time has no major advantage and is carrying much less energy (assuming my calculations are correct).

The data as shown looks like it assumes a constant BC.

The pellet with 1100fps muzzle velocity is at 813fps by 25 yards.

At that point, the BC is probably closer to 0.043 for the remainder of the flight.

[Setarip]

So I just did some calculations on the ballistics of a jsb with .043 BC at 819 fps, and one at 1100 fps with an assumed BC of .015. This may be over simplified, but interesting:

Jsb 0.043 at 819 fps:
Muzzle fpe: 27 fpe
25 yards: 758, 23 fpe
50 yards: 704, 20 fpe
75 yards: 655, 17 fpe
100 yards: 609, 15 fpe

Average velocity: 714 fps

Time of flight: .42 seconds


Jsb 0.015 at 1100 fps:
Muzzle fpe: 49 fpe
25 yards: 813, 27 fpe
50 yards: 657, 17 fpe
75 yards: 533, 11 fpe
100 yards: 430, 7 fpe

Average velocity: 765 fps

Time of flight: .392 seconds

The energy bleeds off very rapidly at 1100 fps and really the .03 secs faster flight time has no major advantage and is carrying much less energy (assuming my calculations are correct).

The data as shown looks like it assumes a constant BC.

The pellet with 1100fps muzzle velocity is at 813fps by 25 yards.

At that point, the BC is probably closer to 0.043 for the remainder of the flight.

That is correct. That's the main problem with the G1 model. I know the G7 model can be used as a constant and be much more accurate (even though the G7 model is not 100% constant either, it's much better. Although g7 is for boat tail spitzers only). I guess this brings up a good point about the G1, how do you accurately account for really long distances when the velocity and BC are ever changing? I believe i have read that long range shooters will use the BC to get really close and adjust from there.


Edit: assuming a BC of 0.043 at 25 yards for the 1100 fps pellet(after a starting BC of 0.015), the average velocity over 100 yards is 875 fps, 100 yard velocity is 650 fps, 100 yard energy is 17 fpe, and the time of flight is approximately 0.343 seconds.  Still only about 0.077 seconds faster, but with a significant decrease in efficiency.

[rsterne]

I think you should go back and read the entire thread.... The changing BC as the velocity drops, and the poor fit of the G1 Model for pellets (and the G7 is much worse), is exactly the reason for this thread and all the measurements.... The BC is only a measure of how the pellet matches the model, whereas the curves I am drawing are the actual Drag Coefficients....

As far as the calculations you did, they are assuming the G1 Model (I just ran them through ChairGun to check), so they DO take into account the reduction in drag as the velocity drops, even though they are using a constant BC.... If the G1 Model was an exact match for the pellet in question, then you would be able to use the same BC for all velocities, and the drag reduction as the pellet slowed would match reality.... The testing we have done on the JSB Heavy suggests that using the G1 Model the BC is about 0.036 at 1100 fps and 0.042 at 819 fps.... Using those values, and others in between, for the appropriate parts of the flight, and the G1 Model, should give you reasonably close results.... With a better model, we should be able to use just a single BC for the entire flight of the pellet from muzzle to target....

Bob