Bob Sterne Reviews HAM FX Hybrid Slugs BC Data
Obviously there’s a great deal of interest in the new FX Hybrid Slugs. This is shared within the HAM Team. So I thought you would be interested to read this discussion on FX Hybrid Slugs BC (Ballistic Coefficient).
This is a “cleaned-up” version of an email discussion between HAM Technical Editor Bob Sterne and HAM Publisher Stephen Archer.
Bob Sterne: Stephen, that post about you shooting FX Hybrid slugs was very interesting. It’s the first test results I’ve seen on the downrange performance of these new slugs.
Stephen Archer: Bob, thanks, I’ll send you the Excel spreadsheet with the test data from Labradar. As you know, we shot the Hybrid Slugs compared to the JSB-manufactured FX pellets for a comparison. As we’ve tested these pellets before, that also serves as a benchmark for these latest tests.
I’ve added the velocity tables at the foot of this post so that those who want all the detail can see it there – SA.
Bob Sterne: Thanks, that’s great. If you can send me details of the temperature, pressure etc, I’ll put these results into Chairgun and calculate the FX Hybrid Slugs BC.
Stephen Archer: The local atmospheric conditions at the time of the test were 35 degrees F (a cold day), 50% Humidity, 244 ft. above sea level, and 29.5 in. Hg local barometric pressure.
Bob Sterne: Now I’ve input the actual test conditions into ChairGun so that the results are properly corrected to ICAO conditions. I used the G1 model for the slugs and the GA model for the pellets.
Stephen Archer: My understanding is that the G1 model applies to pointed pellets or bullets. But the FX Hybrid Slugs have a hollow point – so do some other airgun slugs I’ve seen. So is the G1 model completely applicable in this case?
Bob Sterne: Yes, a hollow point slug design slugs means that using the G1 profile will almost certainly compute a higher BC value than is actually achieved. But it’s the best we have at the present time. That’s why I’ve used the G1 model.
I calculated the BC using the average velocities at the muzzle, and both 40 and 50 yards, and also calculated the BC for EACH shot at both distances, and then averaged the BCs, to see if there were any significant differences.
Although there were significant differences from shot to shot in the calculated BCs, there was no significant difference in the resulting average BC using the two methods.
One thing that stood out was that in the slug testing, two shots were outside the typical variation at 50 yards…
One had a high BC and the other low, compared to the same two shots at 40 yards. At 40 yards the spread in calculated BC for the ten slugs was 10%, while at 50 yards, with those two shots included it was 35%.
Interestingly, the average BC only changed by 1% with these outliers deleted, but they were such a statistical oddity I decided that was the best approach.
Stephen Archer: Yes, we’ve found previously in HAM’s Ballistic Coefficient testing that the FPS readings can become quite erratic further than about 35 – 40 yards – particularly with .177 caliber pellets. This is why we use 30 Yards as a maximum range for this work.
Bob Sterne: I understand why you do that, particularly for .177 pellets. However, LabRadar’s reliable range increases with caliber, and should be about 100 yards in .30 cal.
This .22 cal. data looked excellent to me out to 50 yards, with the exception of those two points that were drastically different than the same shot at 40 yards, which is why I deleted them.
Stephen Archer: I know everyone – including me – would like to have BCs over greater distances. But our experimental testing showed that the variability in readings would make such BCs subject to considerable uncertainty.
I don’t know yet if this variability is due to minute differences in the pellets, or due to the Labradar signal. And there’s enough variability in calculating Ballistic Coefficients already
Bob Sterne: Agreed. Once those two pieces of 50 yd. data were removed, the difference between simply averaging the velocities or calculating the BCs and then averaging those was well into the 4th decimal point.
Regardless, I decided that the best way to calculate the BC for both the slugs and pellets was to average all four numbers, the two 40 yard BCs and the two 50 yard BCs, and report that number. Here are my results…
- 16 gr. FX Pellet, with MV = 942 fps… BC (GA) = 0.035
- 22 gr. FX Hybrid Slug, with MV = 860 fps… BC (G1) = 0.071
The variation between the highest and lowest of the four BCs used to calculate the above was 2.7% for the slugs and 3.5% for the pellets. That means that the numbers above should have a total range of error of about 0.001 for the pellet and 0.002 for the slug.
Therefore the BC (G1) for the FX slug in this test, conducted at a MV of 860 fps, is 0.070-0.072.
Stephen Archer: When we determined the BC of the FX Pellets before, we found a value of 0.032. Same gun, same range, same shooters, same tin of pellets, same Labradar, BUT higher temperature and humidity. Of course, the actual weights of the pellets would have been different for each test. We always use unselected pellets for BC testing to better match real life use.
What do you think about these results?
Bob Sterne: The previous results, as published in the HAM table of BCs was taken at a muzzle velocity of 887 fps, and only over the first 30 yards. In this test, the MV was 943 fps, and I used data at 40 and 50 yards, averaged. I think BC values within 10%, as these are, are quite acceptable, and probably typical for data taken differently.
Stephen Archer: In our practical tests, we found that the pellets were impacting about 8 Inches higher than the slugs at 50 yards. How does that match with your Chairgun results?
Bob Sterne: Velocity is the key factor in trajectory, and the pellets were starting out at 943 fps instead of 860 for the slugs. The pellet should hit higher than the slugs, but by less than an inch at 50 yards. They should come back to the same POI at 100 yards according to Chairgun.
The difference between theory and what you found (about 7 inches) is most likely barrel harmonics. If the barrel is 2 feet long, and it is pointing only 3/32 inch lower with the slugs, that would account for the difference in POI.
Stephen Archer: The FX specifications for the 22 Grain FX Hybrid Slugs BC is 0.080, based on a weight of 22 Grains and at 100 Yards range. The average weight of the slugs we tested was 21.86 Grains – so obviously that will make some difference. And the HAM test was at shorter range – to 50 yards. Starting with a higher Muzzle Velocity would also have given a higher BC.
And we know that Ballistic Coefficient values are subject to a whole raft of variables, including the specific barrel and environmental conditions. FX is well aware of this too, of course, and mentions it on the slugs’ packaging. Of course, that means that the value obtained under specific practical shooting situations could be either higher or lower than the 0.080 value…
How much do Chairgun results for vary with FX Hybrid Slugs BC values of 0.071 and 0.080. Is that really a big deal?
Bob Sterne: No. The difference in trajectory would be negligible within 100 yards. The main difference is that the slug would have about 12% more wind drift than if the BC was actually 0.080 as specified.
Stephen Archer: Bob, thanks a lot for your great input of this. I think that this practical experiment gives HAM readers a lot to think about – especially when read in conjunction with your excellent posts about the BCs of slugs and pellets.
HAM test data with FX Hybrid Slugs:
|Shot||V0||V10||V20||V30||V40||V50||40 Yard BC|
HAM test data with FX Pellets:
|Shot||V0||V10||V20||V30||V40||V50||40 Yard BC|
One thing’s for sure, when it comes to airgun slug Ballistic Coefficients, we’re ALL learning