Airgun Pellet Head Diameters. What Should We Expect?
Today’s analysis of airgun pellet head diameters is a follow-on to our previous story about pellet weight consistency.
Dedicated Field Target and Benchrest competitors have long sorted their pellets. So there’s some experience that sorting by weight – and by head size, too – maximizes downrange accuracy.
Over the years, Hard Air Magazine has published no less than 38 comprehensive pellet test reviews. This forms an un-paralleled amount of independent information about airgun pellet manufacturing consistency across a wide range of manufacturers.
By analyzing all that data, we can make some assessment for what we should expect from the pellet manufacturers’ products. So what should we expect in terms of airgun pellet head diameters when we open a tin of ammo?
Again, let’s give the manufacturers some congratulations!
Given that they’re mass-manufacturing and selling pellets for just a few cents each, the quality is really amazing. It’s clear to any enthusiast that the overall quality and consistency of our pellets has improved greatly over the years.
Of course, head diameter is just one aspect of consistency. But it’s an important one. Let’s see what we can expect….
Overall Pellet Head Diameter Consistency Results.
The head diameter of a pellet determines how deeply it engages with the barrel’s rifling. Variations in this degree of engagement can alter the Muzzle Velocity and result in downrange accuracy issues.
The photograph below shows two pellets of the same type. That on the left is un-fired. The one on the right has been fired. Grooves caused by the pellet’s head engaging the rifling are highlighted by the arrows. (The skirt also shows some grooves, as you can see).
On average, pellets work-out to have head sizes slightly larger in diameter than the nominal barrel diameter. In fact, pellet head diameters average 0.51% larger than the caliber. This makes sense as – if the head diameter were to be smaller than the bore, the head would not engage with the barrel’s rifling and accuracy would be terrible!
The average difference in diameter between the smallest and largest pellets in a tin can be as high as 2.5%. However, typically, it’s more likely to be around 0.6%. But there are some interesting differences between the calibers, as we’ll see below…
Variation Between Caliber And Actual Head Diameter.
Analyzing data from all the HAM pellet test results, we see that the average head diameter is 0.51% greater than the bore diameter.
Generally, larger calibers tend to have head diameters that are proportionally greater, as we’ll see below.
Furthermore, heavier pellets in any specific caliber will tend to have head diameters that are larger. Lighter pellets in a specific caliber tend to have head diameters that are closer to the nominal bore size.
If we now separate the same data by caliber, we see some interesting differences appear.
As the chart above shows, the percentage by which pellet head diameters measure above the nominal bore diameter increases with caliber. So – for example – .22 caliber pellets have greater head engagement in the rifling than do .177 caliber pellets.
It would be reasonable to expect that rifling depth is likely to be greater in larger bore diameters, so that makes sense.
Variation Between Smallest And Largest Head Diameters Of Pellets In A Tin.
As we can see from the next chart, the variation in head diameter between the smallest and largest pellets in a tin tends to be greater in .177 cal than it is in .22 caliber.
That tells us that – on average – there’s likely to be somewhat more benefit to sorting pellets if you use .177 caliber than if you shoot .22 cal. However, in each caliber a good number of pellets tested showed a difference of 0.02 mm – that’s about 8/10 of a Thou – between the largest and smallest in a tin. It’s the outliers that make the difference!
If we take the same data and express it as a percentage, the average difference between largest and smallest is less in .22 caliber than it is for .177. This is basically because a similar level of consistency is being applied to a different diameter. (The larger the bore diameter, the smaller the percentage).
All-in-all, this analysis demonstrates remarkable consistency in airgun pellet head diameters. However, it also demonstrates that – for best results downrange – sorting pellets for consistent head diameters is as valuable as sorting for consistent weights.
For pellet-by-pellet detail, check out the individual HAM pellet test reviews. And don’t forget to thank HAM Tester Doug Wall. He painstakingly measures the head diameter every single pellet using a PelletGage and records the results. That’s 3,800 pellets to date. and counting…
Thanks again Doug!!!