FX Slug BC - Revised at Reply #49

[rsterne]

Miles, I agree that trying to develop an exact drag model to cover a larger range of projectiles would be next to impossible.... What I was suggesting was the approach used by Lapua, where they have a Cd curve for each bullet.... If FX developed that for each of their slugs, and provided the software to use it (like Lapua does)…. they would have something that nobody else does.... customized ballistics prediction for each slug they make....

The limited playing I have done with LabRadar outputs leads me to believe that instead of using the Doppler velocities (which can flop around like a wet fish)…. a far better approach is to use the time/distance data, plot that, generate the t/d equation, and then use the first derivative for the velocity and the second derivative for the deceleration.... Even so, there is such a difference shot to shot, getting perfect results is a muggs game.... and stitching together several sets of data to cover a large velocity range may well prove impossible.... 

When I retire in 2021, I plan to use the LabRadar I was awarded by the AirGunGuild to do some serious work on developing a better drag model (or 3) for pellets.... I think you would agree that the current GA model underestimates the increase in drag in the Transonic, and I think at low velocities as well.... Even if what I come up with isn't perfect, perhaps we can at least have something closer to reality than what we have now.... I have a feeling that the G1 model we use for slugs is not that great either in the Transonic range.... because we typically use slugs with a rather large Meplat, not the Spitzer nose of the G1 drag model....

Anyways, that is my hope.... Perhaps I will be just wasting my time....  …. but I will guarantee you I will learn a lot and have fun doing it.... After all, what more can a Hobby give you than that.... 

Thane, I agree I would have liked to seen a comparison to a pellet of similar mass, at similar velocity.... 

Bob

 

[mcoulter]

…. but I will guarantee you I will learn a lot and have fun doing it.... After all, what more can a Hobby give you than that.... 

Bob, thank you for this reminder!  Love it.  :-) 

[ballisticboy]

Miles, I agree that trying to develop an exact drag model to cover a larger range of projectiles would be next to impossible.... What I was suggesting was the approach used by Lapua, where they have a Cd curve for each bullet.... If FX developed that for each of their slugs, and provided the software to use it (like Lapua does)…. they would have something that nobody else does.... customized ballistics prediction for each slug they make....


Bob

Bob

That is exactly the approach we used on the artillery fire control and, during the last years of my career, we were using a large multi million dollar radar to track bullets of all sizes from 4.5mm to 14.7mm and derive drag laws for each bullet design from the different types of guns. The Lapua data is the way to go, though I have some doubts about some of their data which seems to go up to Mach 5 for their bullets and does not match very well the drag curves we were producing for some of the bullets. All of the trajectory software I use has always used Cd/Mach number curves, not BCs having been designed for artillery shells.

On the bullet trials we were trying to fill in the very short range data using the raw data from the fixed head muzzle velocity radars (like the LabRadar but more powerful), used to obtain the muzzle velocities, but it was not very successful, often producing unrealistic drag curve shapes. We concluded it was because the radar units were often smoothing the data before printing it out making the drag curve shape a function of the smoothing curve, not the data. I suspect that LabRadar does the same thing as most of the data I have seen is just too smooth to be true raw data. Getting true raw data from the fixed head radars is not always straight forward. We always used a minimum of two muzzle velocity radars for all shots which had to agree with each other to less than 1m/s. Getting matching readings with small arms prooved very difficult, the radars being very sensitive to positioning and aiming with the readings often drifting away from each other during 10 round serials.

On the GA drag law Chairgun also has a version of my drag law produced for AA Field pellets using a mixture of prediction and calibration with measured data. The drag law is here.

Mach Cd
0.1   0.8
0.15   0.73
0.2   0.62
0.32   0.44
0.4   0.334
0.5   0.3103
0.55   0.318
0.6   0.327
0.65   0.332
0.7   0.363
0.75   0.404
0.8   0.455
0.9   0.56
1   0.8

You can see the high and low speed data is higher than GA. If you want to use this drag law with the usual BC value for this pellet you will have to either devide the CD values by the form factor value or use the sectional density as the BC value.

[Doug Wall]

As the shooter for all of the HAM BC testing, I just want to throw one thing into the slug design - LENGTH. There have been several pellets that have come through that have been too long to fit into airgun magazines. Sure, some bolt action airguns can handle them, but almost all of the airguns produced these days have magazines. Unless the gun manufacturers decide to produce guns with magazines that will handle them, the pellet manufacturers should probably keep the lengths down to something that they can sell.

[rsterne]

Miles, that is a great set of data.... I don't see it in the latest version of ChairGun that I downloaded, but I have added it to my spreadsheet of drag laws, and plotted it along with all the others from ChairGun…. Photobucket is down right now, but as soon as it is back up, I will add that chart to this thread.... It is VERY different from all the others, particularly under 500 fps.... I wonder why our Diabolo pellets have such high drag at low speeds?....

Bob

[subscriber]

As this is about FX slugs, I would like to suggest FX equip their shooting tunnel with multiple light screens, at 5 meter increments, to the end of their on site 40 m range.  The ones nearest the muzzle could be spaced at 0.5 m to capture "muzzle velocity".  So 8 or 9 screens, including the "muzzle" pair.

Yes, LabRadar is just so convenient; especially to cart out to various indoor or outdoor ranges.  However, for a fixed lab setting or factory, capturing the actual beam breaking time for each pellet at a number of discreet fixed ranges would seem to provide very good raw data for BC measurement.  If the screen spacing were close enough, it would also show if any projectile had marginal stability, because yawing projectiles have more drag, when they travel more obliquely through the air. 

Obviously, setting up such light screens in a shooting tunnel to 100 meters, at 1 meter intervals would be preferred.  However, that would cost a lot more to install than a LabRadar unit; and for some, would seem like a step backwards in technology.   I don't care about backwards.  I prefer accurate information that has not been "smoothed", to cover up shortcomings of a particular system.

[subscriber]

EDIT:  Deleted.  I read CD as BC...

[rsterne]

I agree, for a fixed, laboratory setup, a 100 metre tunnel with screens every metre, would be the ideal setup.... It isn't a "backwards" step if it supplies higher quality data.... Besides, it should show up things like a drag increase due to wobbling or spiraling, and what distance it started at....

Another thing I would like to see is an "easy to use" spreadsheet or calculator that would allow you to input Cd values such as Miles listed above, plus the caliber and weight of the projectile, the muzzle velocity and the wind parameters.... and output the trajectory and wind drift.... Let's forget about using BC's, which are comparing the Cd to some drag model that won't match up.... and use the raw Cd data instead....

Bob

[JungleShooter]

I would like to see is an "easy to use" spreadsheet or calculator that would allow you to input Cd values such as Miles listed above, plus the caliber and weight of the projectile, the muzzle velocity and the wind parameters.... and output the trajectory and wind drift....
Bob

Yupp, I agree.
And whenever I get my second chrono back from the US (I "shot" it with too much voltage 😟) — I want to begin getting data for my preferred gun-pellet combinations.

Because the spreadsheet or program you're asking thankfully exists. 😊
Strelok Pro.

Here're a couple of screen shots (just to see, no valid data).


Matthias

[rsterne]

Matthias.... I didn't realize you could put a custom drag model into Strelok Pro.... Is there a limit to the number of steps (data points) you can input?....

Bob

[JungleShooter]

Bob,

I counted over 200 data fields....!
As this app needs to work for people shooting bullets traversing several mach on their way from the muzzle to target a mile or two out, it seems appropriate to have a bunch of data fields...! 😄

Matthias

[rsterne]

Miles, it was great to hear from you the other day.... I plotted your drag model for airgun pellets and added it to a chart I had of other drag models from ChairGun…. The red line below is the raw data as stated by Miles above....



Miles informed me that his drag model is also available in ChairGun, in the list of Custom Drag Profiles.... You can access it by going the the "Toolbox" Tab, clicking on that and then scrolling down the dropdown menu to "Custom Profile Tools" and then selecting and clicking on "Select Custom Profile".... From the available files, select "MilesMorrisProfile.pro" and click on that.... That will load his drag profile into the "GU" drag function in ChairGun, and you can use it like any of the other drag profiles.... Note that the authours of ChairGun have "adjusted" it to track along with the values of the "GA" profile, so that BC calculations made using it "make sense" instead of being much higher for a given SD.... I suspect that they made a similar "adjustment" to the GA profile so that it produces BCs similar to the G1 profile previously used for pellets....

This information has little to do with the thread on FX slugs.... but I wanted to include it in this thread because that is where Miles (ballisticboy) first published it on the GTA.... He has worked in external ballistics of projectiles for the last 40 years, by the way.... 

Bob

[rkr]

As the shooter for all of the HAM BC testing, I just want to throw one thing into the slug design - LENGTH. There have been several pellets that have come through that have been too long to fit into airgun magazines. Sure, some bolt action airguns can handle them, but almost all of the airguns produced these days have magazines. Unless the gun manufacturers decide to produce guns with magazines that will handle them, the pellet manufacturers should probably keep the lengths down to something that they can sell.

That is a very important limitation indeed. If they don't fit the magazine people won't shoot them. It seems that 10-11mm is the length limit for mags. In practice that means that for a good BC we should go down in caliber to allow longer bullets. Unfortunately .177 twistrates are too slow for that in general leaving .172 as a working solution.