Jim studies pellet design to explain why they behave the way they do in different air rifles
Is there a simple answer to the muzzle energy/ pellet weight question? Ask Jim Tyler
The big problem with trying to make sense of the working of different types of airguns, and especially in fault finding, is that everyone is looking for simple answers to very complex issues. A prime example is cold weather pellet point of impact (POI) shift, which many seem to ascribe to the grease on the spring and hammer of a PCP, or the spring, piston and seal of a springer, thickening and robbing the hammer or piston of energy. Simple, easy for anyone to understand, and wrong. The increased viscosity of cold grease will undoubtedly have a small effect on the air rifle, but what effect there is will be dwarfed in both the PCP and springer, and for different reasons.
The potential energy (to drive the pellet) in a PCP’s air depends on its pressure and volume, and its temperature. For any given pressure and volume, higher temperature means higher muzzle energy, and vice versa, and that will move the pellet POI; down in winter, up in summer.
The spring rifle is the complete opposite, and the POI is most likely to rise in winter temperatures, fall in summer, which it does because the piston seal shrinks in cold conditions, swells in hot, hugely affecting the friction between it and the cylinder wall, not only changing the muzzle velocity but, in the worst cases, the point in the recoil cycle that the pellet exits the muzzle, which can cause huge changes in POI, often reported as in the region of an inch at 30 yards.
“… The relationship between pellets and spring air rifles truly is fiendishly complicated …”
Pellet POI shift is but one example of why a simple ‘one size fits all’’ solution rarely applies to any aspect of the workings to any air gun
PCPS AND PELLETS
It is widely accepted wisdom that PCPs achieve higher muzzle energies with heavier pellets, and that the reason is that the heavier pellets travel more slowly up the barrel, and have more time to gain energy from the air. Unfortunately. It isn’t quite that simple, as some lighter pellets can give higher muzzle energies than heavier ones.
We can, however, qualify the statement that PCPs achieve higher muzzle energy with heavier pellets by preceding it with ‘All other things being equal’. That means pellets having the same head and skirt diameters, the same skirt thickness, and the same hardness, because all of these (which combine to dictate pellet start pressure) can affect muzzle energy every bit as much as pellet weight.
I tested a range of pellets through my S510, the results of which can be seen in Table One.
Arranged by pellet weight, it’s clear that the muzzle energy is influenced by far more than mere weight, and we find the same if we arrange the pellets by start pressures, size, hardness or any other single characteristic, because the muzzle energy depends on all the characteristics, and not all of them always assuming the same degree of importance.
So, how do we read those figures? The lightest pellet does indeed give the lowest muzzle energy, and the heaviest the highest muzzle energy, but in between the two extremes it’s a jumble, so let’s try to make some sense of it.
If we separate out the three low start pressure pellets (Air Arms Field and Express, and the Falcon Accuracy Plus), then we find that their muzzle energy and mass do line up. If we separate out the two medium weight, high start pressure pellets (Superdome and H&N Field Target Trophy), then their mass and muzzle energy figures also line up, and the same goes for the two heavyweight high start pressure pellets (Supermag and Bisley Magnum).
All other things being equal’, heavier pellets do indeed give higher muzzle energies in PCPs.
SPRINGERS AND PELLETS
If the relationship between PCPs and pellets is complex, that between springers and PCPs is fiendishly so. In the 2017 Summer edition of Airgun World, I described a series of experiments in which I swapped the ‘power plants’ (cylinder, piston, spring, guides and preload washers between my TX200 and TX200HC, in an effort to discover how much energy pellets gained in the extra 3” of barrel of the full length rifle, and unexpectedly found that not only did each pellet have its own individual acceleration profile, but that each of the strokes of the two rifles (96mm and 85mm) also had their own pellet acceleration profiles. The relationship between pellets and spring air rifles truly is fiendishly complicated.
In an attempt to simplify the relationship between pellet and springer, many believe that lighter pellets tend to give higher muzzle energies but, even more than in the case of the PCP, that desperately needs qualifying with the preface ‘All other things being equal’.
In an effort to throw some light onto the relationship between the springer and pellet, I recorded the shot cycles of my .177 TX200 with nine pellets, and the results can be seen in Table Two.
Unlike my results with the PCP, the lightest pellet does not give the highest muzzle energy, nor the heaviest the lowest, and in fact the only thing in common between the PCP and springer results is that the pellets between the two extremes of weight are a jumble! Clearly factors other than pellet weight are at work, and the prime suspect is pellet start pressure.
In order to make sense of the results, I arranged them by muzzle energy, and created the bar chart ‘Chart One’.
Now, perhaps, it all begins to make sense. Of the four highest muzzle energy pellets, three (AA Express and Field, and Falcon Accuracy Plus) have low start pressures, so the piston accelerates them in the barrel for longer. The fly in the ointment of the four highest energy pellets is the Super-H-Point, for which I don’t have a start pressure, but a close examination of its skirt reveals that it is markedly thinner than that of its stablemates the Hobby and Superdome, so it will definitely seal at a lower pressure, and I strongly suspect that its muzzle energy is due to a combination of low start pressure and very light weight.
Moving on to the HW Field Target Special, the Hobby and Superdome, the FTS is the heaviest but has the lowest start pressure and the highest energy, the Hobby and Superdome have similar start pressures but the Hobby is lighter, so has higher muzzle energy.
Finally, the heavyweight Bisley Magnum and SuperMag have the lowest energy, with the lower start pressure of the Bisley Magnum edging it over the lighter weight of the SuperMag.
WHERE DOES THE ‘LOST’ ENERGY GO?
In recording the recoil and surge of the TX200 shot cycles with the nine pellets, I was able to get a handle on the piston bounce of each shot, which allowed me to compare the muzzle energy and piston bounce for each pellet. The result can be seen in Chart Two.
The piston bounce appears to be dictated not only by the pellet weight and start pressure, but also by the pellet acceleration, and possibly the energy vented through the muzzle following pellet exit, or its timing. .
The three lowest start pressure pellets generate the least piston bounce, followed by the two lightest pellets (Super-H-Point and Hobby), with the remaining four lining up more by start pressure more than mass.
I always select pellets on the basis of accuracy ahead of any other consideration.
I use Air Arms Express in my TX200 for the flatter trajectory rather than muzzle energy considerations.
Anecdotal evidence suggests springers reform mildly damaged pellet skirts and shoot them OK, but not PCPs.
‘All other things being equal’, heavier pellets do indeed give higher muzzle energies in PCPs.
Results from my TX200 show that there is anything but a simple relationship between pellet weight and muzzle energy.
Results from my S510 show that there is no simple relationship between pellet weight and muzzle energy.
The main destination for lost air energy is piston bounce.
Arranged in order of descending muzzle energy, these results from my TX200 suggest that far more than pellet weight is at work.