Author Topic: Ballistics Coefficients Discussion (Read 58395 times))

Bwalton · « **Reply #120 on:** April 13, 2015, 09:01:17 PM »

Well there is not much that I can add but this pellet data base of some old pellets that are not made any more as well as today's pellets. I would like to try a few of them, one pellet that I did try that to me beat out the JSB pellets was the Logun Penetrators.

http://www.photosbykev.com/wordpress/2009/01/20/air-rifle-pellet-database/

Pellgunfun · « **Reply #121 on:** April 13, 2015, 10:42:20 PM »

I've considered getting some Eun Jin 28 gr pellets, and removing the skirt to see how that performs. I'd leave the top section of the wasp waist to give it sort of a boat tail profile. It may not do well, but ya never know till you try it, right.

I found a couple pictures where a guy had a mold that would produce lead pellets exactly like what I just described. He never had a chance to go test them out and the thread just went cold, so I have no idea if it worked well or not.

I'll include a picture I found of the pellet and mold. I found these over at http://castboolits.gunloads.com/

The pellet on the left is the one that looks like the Eun Jin 28 gr with the skirt removed. Below there are two images of the mold used to make the pellet on the left. The pellet on the left is about 23 gr 22. cal, the middle pellet is .20 cal 15 gr, and the ball is .185 round ball about 9.5 gr.

Actually the pellet in the middle looks like it could be a real performer:-)

rsterne · « **Reply #122 on:** May 23, 2015, 04:58:57 PM »

It's been a while since I added any data to this thread.... Today I received 4 sets of data from jhm757 on the brand new 34 gr. JSB King Heavy, shot from his SynRod.... After correcting to NTP, here are the BC's for that pellet, using the G1 Drag Profile.... Average velocities between the 1 yard and 25 yard Chrony distances are shown....

678 fps - 0.0327
770 fps - 0.0441
832 fps - 0.0477
880 fps - 0.0457

The muzzle velocity for the 880 fps data was 917 fps and if we start at that velocity, we could use a BC of 0.0457 out to 17 yards, then change to 0.0477 out to 40 yards, then use 0.0441 out to about 65 yards.... That averages out to using about 0.046 out to 65 yards with a MV of 917 fps.... After that, the BC seems to decrease, but bear in mind this is just one set of data, from one gun.... Here is a chart showing how the new pellet compares to the other JSBs I have data for....

While a BC of 0.046 is pretty respectable, the Drag Coefficient, from this set of data, seems to show that the new pellet has more drag than the other JSBs we have data for.... Anyone who has additional data can send it to me to be added to the database.... Don't forget to record the temperature, altitude, barometric pressure, and humidity.... So far, we seem to be showing the same trend as the other JSB Exact series pellet towards least drag (and best BC) at around 800-850 fps....

Bob

Scott Endler · « **Reply #123 on:** May 23, 2015, 05:08:16 PM »

Why would the BC be so much worse at 700 fps?

rsterne · « **Reply #124 on:** May 23, 2015, 05:15:03 PM »

I have seen the BC with JSB pellets decrease at low velocities before.... You will notice that we don't have data down at 678 fps for any of the others tested (yet).... The best set of data I have (most complete) is for the AA 16 gr., and you can see the drag increasing below about 830 fps, but the lowest data point is 740....

As to the REASON for the BC to decrease at low velocities, I don't have the answer, but that is part of what I am trying to do here is establish a new Drag Model for pellets, in particular for JSB Exacts.... Even the G1 Drag Profile has higher drag below 600 fps....

Bob

rsterne · « **Reply #125 on:** June 01, 2015, 10:39:51 PM »

All of this is about to become obsolete, if you can afford it.... A new Doppler Radar for ballistic use, called LabRadar, is coming on the market, supposedly priced at $559.00.... It will work out to about 100 yards with a flat based .30 cal, less range with a smaller caliber (I have heard about 40 yards with a .177), and can be set up to give a plot of velocity vs. distance in increments as small as about 1 yard or less.... Here is a plot of a 34 gr JSB King Heavy, starting at 785 fps, out to 65 yards (which I assume is where it lost acquisition).... The plot of actual measured velocities is laid over a constant Cd of 0.251, and the agreement is amazing....

http://www.network54.com/Forum/79537/message/1433191509/Lots+of+good+data+there%2C+Harry

I would expect that at transonic velocities the Cd would no longer be a constant, but this device should be able to plot the entire Drag Model for any pellet you want to shoot, at any velocity.... By using a few velocity increments, say 100 fps steps, it should be possible to plot the entire Drag model for a given pellet.... We are on the verge of a complete change in how we can measure downrange velocity and BCs.... All it is going to take is deep pockets.... For now, that leaves me out, unfortunately....

Bob

sixshootertexan · « **Reply #126 on:** June 01, 2015, 11:43:34 PM »

Interesting that radar can pickup such a small object like that.

rsterne · « **Reply #127 on:** June 01, 2015, 11:45:47 PM »

and the best part is.... Made in Canada....

Bob

Pellgunfun · « **Reply #128 on:** June 02, 2015, 10:34:56 AM »

Quote from: rsterne on June 01, 2015, 11:45:47 PM

and the best part is.... Made in Canada....

Bob

By an inventor that was probably educated in America

rsterne · « **Reply #129 on:** June 02, 2015, 01:14:42 PM »

Bit of an arrogant comment, when you consider that Radar was invented by the British, don't you think?....

Bob

QVTom · « **Reply #130 on:** June 02, 2015, 01:51:44 PM »

LOL, Doppler Radar chronographs have been around for years although they haven't been available to the hobbyist until now. Some hobbyist. I've read about military surplus chronys before but it seems the US government no longer auctions of stuff like they used to. And yes, the credit goes to the British.

Tom

Pellgunfun · « **Reply #131 on:** June 02, 2015, 02:31:16 PM »

Good grief, I was only kidding. Didn't you notice the smiley face? Thanks for the history lesson, but I already knew that.

rsterne · « **Reply #132 on:** June 03, 2015, 06:34:52 PM »

The investigation of Cd changing with velocity using LabRadar on the Yellow continues.... http://www.network54.com/Forum/79537/message/1433252500/Old+Kings+at+high+speed+--+interesting

The Cd for the new 34 gr. and old 25 gr. Kings is similar at similar velocities, as expected.... and the Cd increases at higher velocities.... Seems like they are working on the same thing I am.... parallel discovery is always interesting.... Like I am finding, the actual Cd values are slightly higher than the G1 or GA profiles in the subsonic region.... They haven't played with transonic or supersonic yet (at least not in that thread), so they may be in for a shock when they do, as I found the drag skyrockets....

Bob

Scott Endler · « **Reply #133 on:** June 06, 2015, 08:24:49 AM »

Where in these graphs is showing the drop in BC below 750fps that you have been measuring?

rsterne · « **Reply #134 on:** June 06, 2015, 12:47:49 PM »

Most plainly shown in Post #83, but there are other examples as well if you read back.... Michaelthomas has studied it quite a bit with Exacts and 16 gr. AA pellets for his 25 yd. Benchrest shooting, there is a sweet spot where the wind drift is at a minimum somewhere in the 800s....

I notice that the testing with the LabRadar, so far, hasn't shown it, and in fact the Cd is pretty much a straight line below about 750 fps (in .25 cal), so there is still much to learn.... The most important things are still to be learned, exploring the Cd at lower and higher velocities to fully develop the Drag Profile....

Bob

Scott Endler · « **Reply #135 on:** June 06, 2015, 07:50:38 PM »

That is what is a little surprising. CD is continually improving at lower velocities but BC is known to get worse again below 750 fps.
.
Does the formula for BC have any multiplying factors based on the energy?

rsterne · « **Reply #136 on:** June 06, 2015, 09:04:22 PM »

If you look at the G1 drag model, you will note that the Cd is actually higher at low velocities, it has a minimum at 600-650 fps.... The BC is only a valid concept when you compare one bullet to a standard with a known Cd profile, such as the G1.... That is why if you use the G1, GA, GS, or G7 models, you get a different BC each time.... The idea here is that I would like to develop a new drag profile that is closer to reality for airgun pellets than what we have been using (G1 or GA).... None of them account for the huge increase in drag in the transonic region we get with Diabolo pellets....

The BC for a bullet that exactly matches the G1 model is 1.000.... It has a Sectional Density of 1.000, so if you have an SD of 0.15, then if the Form Factor is the same (1.000), you would expect a BC of 0.15 as well.... If the Form Factor is 2.000, with an SD of 0.15, the BC would be 0.075.... Use a different model, everything changes....

Bob

MicErs · « **Reply #137 on:** June 06, 2015, 09:21:29 PM »

Have you thought about sabots? Just a question.

rsterne · « **Reply #138 on:** June 06, 2015, 10:08:05 PM »

Nope.... never seen any accurate results with them....

Bob

MicErs · « **Reply #139 on:** June 06, 2015, 10:45:43 PM »

Quote from: rsterne on June 06, 2015, 10:08:05 PM

Nope.... never seen any accurate results with them....

Bob

They are popular in the muzzle loading world. I have no idea one way or another about how they might fit into the air gunning world. They probably won't. Also shotgun slugs are often carried in a sabot. This allows more powder to burn in the same space thus pushing a longer slimmer and sometimes lighter projectile to a higher velocity. Again I have no idea how this might apply in the airgun world. Hypothetically a gun designed to shoot a 9mm pellet could shoot a .257 which weighed the same in a sabot. It would achieve the same muzzle velocity but it would have a much better BC. Bob, this is just me rambeling... maybe it is worth thinking about.

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Ballistics Coefficients Discussion

Author Topic: Ballistics Coefficients Discussion (Read 58395 times))