The Big HAM Slugs Test. Part Three. Airgun Slug BC Variability And Projectile Weight
In part one of this Big HAM Slugs Test, we talked a little about airgun slug BC variability. In this post, we’ll cover that topic in more detail as it relates to projectile weight. We’ll also make some comparisons with BC values generated in previous HAM testing and manufacturer’s claimed specifications.
There’s additional background information in HAM Technical Editor Bob Sterne’s post on The External Ballistics of Slugs in Airguns.
Causes Of Airgun Slug BC Variability – Slug Weight
We know from many HAM test reviews that all slugs and pellets vary in weight from one to another. To be clear, this is not a strike against airgun ammunition manufacturers, it’s just a fact of manufacturing life. Any two samples of ANY product will be slightly different.
If you don’t believe me, just try driving a different example of the same car as you drive regularly. You’ll soon notice the difference!
For this Big HAM Slugs Test, we shot slugs directly from the tin. There was no sorting.
As you can read in individual, comprehensive, HAM test reviews, we have found that slug weights can vary from 0.4% to 1.5% across a batch of slugs from the same manufacturer and type. This is really very good consistency, however it can make a noticeable difference to the actual BC values achieved.
Below we see the weight variability results from an analysis of 50 samples in the comprehensive HAM review of JSB Knock Out slugs.
Now – perhaps surprisingly – variations in weight DO NOT make a difference to a slug’s BC value. So long as the Muzzle Velocity stays the same, that is.
However, in practice differences in weight will affect the Muzzle Velocity. (Heavier slugs will be slower, of course) and that change in FPS DEFINITELY DOES affect the BC.
Note, also that we’ve used the manufacturer’s claimed weights for the BC calculations. HAM testing also proves that the average projectile weight can often not match with the manufacturer’s specs. There’s another way in which BC calculations can be skewed!
Comparison Of Airgun Slug BC Variability With Pellet BC
So let’s make the – not unreasonable – assumption that differences in weight between individual projectiles are primarily responsible for differences in the velocities achieved by both slugs and pellets.
Looking at the test results from the Jumbo Monsters, we find an Extreme Spread of 13 FPS in velocity at 30 Yards. It was 9 FPS for the JSB Knock Outs.
For simplicity of comparison, let’s assume an Extreme Spread of 10 FPS for both types of JSB projectiles. Now let’s apply that 10 FPS difference to the ACTUAL test results we obtained with the KalibrGun Cricket 2 Tactical.
Again, for simplicity, let’s take the BCs achieved on test, then reduce the 30 Yard velocity by 5 FPS. We can also increase the 30 Yard velocity by 5 FPS. That gives our Extreme Spread of 10 FPS in velocities with the actual test results in the middle.
Now we can calculate the 30 Yard BCs for 30 Yard velocities at our chosen Extreme Spread of 10 FPS. Here’s the results…
As we can see, the BCs for the actual average test velocities are 0.047 for the Monsters and 0.077 for the Knock Outs.
Had the 30 Yard average velocities been just 5 FPS less, however, the values would have been a BC of 0.043 for the Monsters and a BC of 0.070 for the Knock Outs.
With 30 yard average velocities of 5 FPS more than the actual test numbers, the BCs would become 0.051 for the monsters and 0.088 for the Knock Outs!
Here’s the same data in a table for those who prefer to look at numbers rather than charts…
|Projectile||Actual 30 Yard |
|Actual Test BC||30 Yard|
Velocity - 5 FPS
|BC For 30 Yard |
Velocity - 5 PFS
|30 Yard |
Velocity + 5 FPS
|BC For 30 Yard |
Velocity + 5 FPS
|JSB Knock Out Slugs||834 FPS||0.077||829 FPS||0.070||839 FPS||0.088|
|JSB Jumbo |
|824 FPS||0.047||819 FPS||0.043||829 FPS||0.051|
So it’s clear that relatively small changes in velocity produce quite large changes in Ballistic Coefficients. This applies to both slugs and pellets.
However – if we compare the BCs generated from this exercise – we find that the difference between the BCs at the +5 FPS and -5 FPS values are 17.0% for the pellets and 23.4% for the slugs. That 23.4% is more than a third greater than 17.0%.
In other words, the variability in BCs for slugs is actually 37.6% more sensitive than for the pellet BCs! (If only due to the different formulae used for the different projectiles, the G1 model for slugs and the GA model for pellets). WOW! That’s another big learning!
The implication of this is clear. For most consistent actual BC performance, slugs need to be sorted by weight.
Other Causes Of Airgun Slug BC Variability
Of course, projectile weight is far from being the only cause of difference in measured BCs. There’s so many other factors, including the inherent power of your airgun, it’s barrel length and actual bore diameter.
Then there’s environmental factors, such as temperature, relative humidity and the elevation above sea level where you’re shooting.
In HAM standard BC testing, we use an FX Impact Mk 2, shoot at temperatures around 80 degrees F and at a height of 244 Feet above Sea Level.
This Big HAM Slugs Test was made using a KalibrGun Cricket 2 Tactical air rifle, shooting at a temperature of 70 degrees F and at an elevation of 1,250 Feet above Sea Level. (Although the relative humidity is similar at around 75%).
And obviously, we’re shooting different, unsorted, projectiles with all the implications this generates for variability around weight and body diameter.
So you would expect to see differences in the BCs from this test to those in previous HAM tests. Likewise against the BC values generated by slug manufacturers. (And let’s face it, the manufacturers are likely to quote the best BC figures they can generate, rather than the worst!).
Airgun Slug BC Variability – Different Test Results
Now we will compare the BC test results from this latest Big HAM Slugs Test with those from previous HAM “standard” BC tests and the claims made by the manufacturers.
Sorry, but there’s no way this cannot be complicated where 13 different slugs plus one “control” pellet are involved! But here goes…
First we have a graphical representation of the results.
And now the same information as data in a table. Phew!
|Ammo||This HAM Test||Previous HAM Test||Manufacturer’s Spec|
|JSB Monster Redesigned Pellets||0.047||0.039||N/a|
|Daystate Howler Slugs||0.069||0.060||N/a|
|H&N Slug HP 21 Grains .217 Dia||0.072||0.070||0.091|
|FX Hybrid Slugs||0.072||0.070||0.080|
|H&N Slug HP 23 Grains .217 Dia||0.078||0.075||0.094|
|ZAN 23 Grain Slugs||0.070||0.072||0.075|
|NSA 24.8 Grain Slugs .217 Dia||0.070||0.073||0.080|
|H&N Slugs HP 25 Grains .217 Dia||0.079||0.077||0.100|
|JSB KnockOut Slugs .216 Dia||0.077||0.086||0.084|
|ZAN Slugs 25.5 Gr||0.070||0.075||0.082|
|H&N Slugs HP 27 Grains .217 dia||0.077||0.085||0.095|
|NSA 27.5 Grain .217 Dia||0.070||0.080||0.090|
|ZAN 28 Grain Slugs||0.072||0.080||0.090|
|H&N Slugs 30 Gr .218 Dia||0.071||0.091||0.106|
Given the range of BC variability caused by weight differences alone, I’d say that these numbers are relatively consistent.
One clear conclusion is that H&N is using a much more powerful air rifle to generate their Ballistic Coefficients. That’s probably why their claimed values are significantly higher than the HAM test data.
Again, I’d like to thank Matt Coulter for all his help in this test!
For Part One of this Big HAM Slug Test, click here
For Part Two, click here.
Next, we’ll move on to accuracy testing…