Pellet/Slug Labeling, yet no BC mentioned.

[Alan]

I was nice to read the recent thread explaining the labeling on JSB pellet/slug tins. Glaringly missing is any mention of BC, and JSB isn't alone. Those few who do mention BC on their label, still use the G1 profile, which is grossly inaccurate for pellets, and even falls way short for slugs. While the RA4 profile is close for the latter, but even this profile leaves a lot to be desired. Bob Sterne (et. al.) have developed better profiles, some of which have been available for nearly 10 years. But here we are in 2024, with inadequate BC profiles essentially across the board, while our firearm compatriots have them all!

 

[hameditor]

@Alan, actually JSB KnockOut slugs do carry the BC value on their labels. This is shown in one photograph and included in the "Detailed Descriptions" section at the end of the post The Easy HAM Guide To JSB Pellet Tin Labels - Hard Air Magazine

As you know, HAM has the world's largest publicly-available list of airgun pellet and slug BCs at Ballistic Coefficients

However, I agree with your basic point that Ballistic Coefficient data should be much more widely available for airgun ammo - and that it should be as accurate as possible. With regard to slug BCs, some manufacturers have tried using the GA model too - this gives even more wildly optimistic values that are great for marketing but not real-world use.

 

[Alan]

And that is the point. If you do the necessary calculations for the ambient conditions, none of the current BC profiles work. As you allude to, G0, G1, and even RA4, do not give accurate results.

I do own an FX Ballistic radar, and I use the RA4 profile as it is the closest to being correct, if one could call it that. But, with the FX BC being a given, holdovers don't start to fall apart until about 150 yards. In my case, that is close enough. This said, wouldn't it be nice to have a pellet/slug-real-world BC number?

 

[Miles_M]

We have tried to address the question of slug reference drag laws with the SLG0 reference drag law, but it needs verification and calibrating with firing data. Boattailed slugs will need a different reference drag law.

Creating a reference drag law is the easy bit, getting manufacturers and shooters to use them is much harder. The majority of the available trajectory programs are still stuck with G1 and maybe RA4. GA has been available for many years for pellets, but is also not widely available. The fact that some manufacturers should try to use GA for slugs shows a lack of the basic understanding of BCs in parts of the airgun ammunition industry.

The FX ballistic radar in its current form does not appear to be suitable for calculating BC values. There does not appear to be a way of inputting the atmospheric conditions when the unit is being used, so it cannot be expected to produce an accurate BC value. A recent test of one gave wildly optimistic values of BC for a particular design of pellet, which I am currently trying to look into. There also does not appear to be a way of getting the raw data out of the FX which you can to some extent out of the LabRadar units. Without the raw data, there is no way of checking the derived results shown on the screen at your chosen ranges. Even with the Labradar, getting good, accurate drag data is not easy and needs careful setting up and experimental technique, particularly as both the FX and the LabRadar are fixed head doppler radars.

In the end, if you really want accurate long range trajectory data, you need to go away from BCs and use purpose drag laws as artillery projectiles have been doing for over 60 years. Of course, for that you need not only the drag law but also the trajectory model to use it, and not many people have those.

 

[hameditor]

Miles, thanks for your input, as always! Agreed, using Labradar requires careful experimental technique.

That's why Doug Wall and I try so hard for consistency when we are producing the HAM ballistic coefficients. We extract the raw FPS data from the unit and then enter that into a spreadsheet created by Bob Sterne especially for the purpose. That actually generates our BCs.

 

[Alan]

Miles, your input on ambient conditions brings up another issue, and that is twist rates. Where I live hovers around 3,800 FAS, with summer temps averaging over 95°F. The fastest twist FX offers in 1:18 (.25 caliber). Even with lighter slugs (26.8 gr. in my case), it is necessary to keep the velocity about ≈990 FPS in order to keep them stable. Yet, the calculated twist rate says they should be stable.

 

[Miles_M]

The FX ballistic radar in its current form does not appear to be suitable for calculating BC values. There does not appear to be a way of inputting the atmospheric conditions when the unit is being used, so it cannot be expected to produce an accurate BC value. A recent test of one gave wildly optimistic values of BC for a particular design of pellet, which I am currently trying to look into. There also does not appear to be a way of getting the raw data out of the FX which you can to some extent out of the LabRadar units. Without the raw data, there is no way of checking the derived results shown on the screen at your chosen ranges. Even with the Labradar, getting good, accurate drag data is not easy and needs careful setting up and experimental technique, particularly as both the FX and the LabRadar are fixed head doppler radars.

Some of the above is not accurate. Apparently, it is possible to input the atmospheric conditions into the FX radar for BC calculation using the mobile app. It would also appear that there may be a way to obtain the raw data from the radar so that you can do the calculations yourself, I am waiting for confirmation on that one.

Unfortunately, none of it helps to find the reason for the wildly inaccurate BC values which were reported on another forum.

 

[Spinach]

Miles, thanks for your input, as always! Agreed, using Labradar requires careful experimental technique.

That's why Doug Wall and I try so hard for consistency when we are producing the HAM ballistic coefficients. We extract the raw FPS data from the unit and then enter that into a spreadsheet created by Bob Sterne especially for the purpose. That actually generates our BCs.

I would love to see that spreadsheet.
Bob's work is the stuff of legends.

Last year I started swaging my own slugs, just as a addition to the hobby.
The Labrador and Strelok pro and a bit of manipulation help, but sometimes I end up confused.
Then I start all over again .

 

[Alan]

Just for the record...

You can set the app to collect local weather conditions. While that is of use, the weather here in SE New Mexico can vary widely over a small statistical area. However, it is my understanding this specific issue is in the works to allow manual input.

 

[Miles_M]

Miles, your input on ambient conditions brings up another issue, and that is twist rates. Where I live hovers around 3,800 FAS, with summer temps averaging over 95°F. The fastest twist FX offers in 1:18 (.25 caliber). Even with lighter slugs (26.8 gr. in my case), it is necessary to keep the velocity about ≈990 FPS in order to keep them stable. Yet, the calculated twist rate says they should be stable.

It depends which calculator you are using for stability. Most of them that I know of cannot take account of the effects of hollow noses or hollow bases, so the assumed centre of gravity position is not necessarily correct. They also use fairly sweeping assumptions for their calculations, so they are only meant to give a general figure for the necessary twist rate for stability. If you want something better, you are into fairly comprehensive programs which accurately calculate the centre of gravity position and can calculate the aerodynamic moments of the slug.

What is correct is that the gyroscopic stability of a slug decreases as speeds get close to the speed of sound, the same as most other spin stabilized projectiles. The change in stability can either reduce or increase group sizes, depending on the degree of stability at slower speeds. If a slug has more gyroscopic stability than needed at the lower speeds, then reducing the stability by increasing the speed may be beneficial. Increasing the speed of a slug will not make an unstable slug suddenly stable, but it may improve group sizes from both the flight point of view and from the pellet gun interaction.