Do lighter pellets always mean faster velocity, and flatter trajectory? It ain’t necessarily so, says Jim
People who use lightweight pellets generally seem to do so because they want the fastest muzzle velocity, in the expectation of it giving the flattest trajectory, but one does not necessarily guarantee the other. Pellet mass (weight) is one of the factors that contributes to the pellet’s Ballistic Coefficient (BC), which is a measure of how well the pellet maintains velocity in flight and, all other things being equal, greater pellet mass equals higher BC, and hence higher velocity retention. Lightweight pellets might have the highest muzzle velocity, but if they shed velocity in flight too quickly, they might not give the flattest trajectory out to any great range.
Pellet drop is largely – although not exclusively – a function of flight time. Over short ranges, high velocity lightweight pellets with poor BC may well get to the target ahead of heavier ones with better BC, but at some point, the heavier pellet with better velocity retention can catch the lightweight up, and then overtake it, so it arrives at the target sooner, and drops less.
If you missed last month’s Airgun World, you might be forgiven for thinking that lightweight pellets will create less recoil. In the case of the PCP then yes, it’s true, but the difference between very light and very heavy pellets is so little, in the order of under one tenth of a millimetre, as to be inconsequential for the vast majority of people and purposes. In the case of recoiling spring airguns, though, the effect of pellet mass on the recoil cycle is tiny compared to that of the start pressure of the pellet. The only point we can safely draw regarding pellet mass and spring airgun recoil is that, for two pellets with similar start pressure, the lighter will create MORE recoil, not less, and the heavier pellet will create less recoil, at the expense of more surge.
My experiments strongly suggest that spring airguns are far more affected by pellet start pressure than they are by pellet mass, not only in the effect on the recoil cycle, but also in how pellet mass and start pressure help to determine muzzle energy.
There is a general tendency for pellets with low start pressures (typically characterised by very thin skirts and soft lead alloy) to produce higher muzzle energy than pellets with thick and/or hard skirts, and this is due to the pellet skirt deforming, conforming to the rifling in the breech, and hence sealing, at lower pressure, so that the pellet and piston travel in the same direction – the ‘cylinder pulse’ – when most energy is gained by the pellet, for longer.
Pellets with thick and/or hard skirts will not form a seal until the cylinder air pressure is much higher, so they start to move later in the compression stroke, reducing the time that they and the piston are travelling in the same direction. The reduction in the cylinder pulse with hard pellets is counteracted to a degree by the higher air pressure and internal energy when they do start to move, but on the whole, the longer cylinder pulse usually gives the higher muzzle energy.
To my knowledge, the only way
“In the UK, we tend to try to run our airguns to give the highest velocity that’s safely within the UK muzzle energy limit”
for a high start-pressure pellet to deliver higher muzzle energy than a low start-pressure pellet of similar mass is for the air energy and pressure to be boosted by autoignition (dieseling), which occurs at something over 200psi, at around 85% into the compression stroke, and provides a pressure ‘spike’ sufficient to start the pellet earlier in the compression stroke. This is what was happening in many spring airguns in the 1980s, when people were deliberately getting lubricant into the compression volume to promote dieseling, and the pellet that consequently gave the highest muzzle energy was the high start pressure, 7 grain (gn) RWS Hobby.
Combine a low start pressure with low weight, and the pellet exit can be so early that it reduces cylinder air pressure greatly during the initial stage of piston bounce, leading to reduced piston bounce, and hence recoil surge, and a very gentle piston final landing. The pellet that does this best is the 7.3gn Falcon Accuracy Plus, as can be seen in the illustration ‘Piston bounce’.
Experiments with PCPs suggest that they are far less susceptible to pellet start pressure than spring airguns, but more affected by pellet mass, to the extent that PCPs have a general tendency to produce higher muzzle energy from heavier pellets, which is the opposite of spring airguns.
The relationship between pellet mass and PCP muzzle energy is not infallible, as a test of ten pellets shot through my Air Arms S510 revealed. True, the two heaviest pellets produced the two highest muzzle energies, but the third heaviest produced the second lowest muzzle energy.
Pellet start pressure may or may not play a part, but it’s not dominant as in a spring airgun, because the pellets with the highest start pressures produced the two highest and two lowest muzzle energies. Some other factor must be at work, and the most likely culprit seems to be blow-by. The illustration PCP, pellet mass and muzzle energy shows the masses of the ten pellets in the test, but not the hardness of the pellet skirt, so let’s put that right by recognising that the three lowest muzzle energies were recorded by the three pellets with the hardest skirts. The hardness of the skirt may be allowing a degree of blow-by, but it is matched, of course, by the hardness of the rest of the pellet, which may increase friction in the barrel, adding to energy loss.
The illustration ‘PCP velocity and pellet mass’ shows that, whilst using lightweight pellets in a PCP might cost the user muzzle energy,
it usually (not always) results in a higher muzzle velocity. The average weight of the five pellets that gave the highest velocity was 7.8 grains, against the average 10.34 grains of the five that gave the lowest muzzle velocity.
So, for a PCP, is it better to go for a lightweight pellet that gives higher muzzle velocity in the cause of a flatter trajectory, or the pellet that gives the highest muzzle energy? The answer is, of course, neither, but to select pellets according to their accuracy for the intended purpose.
Of the pellets in the test, one of the two 8.44 grain pellets – the Air Arms Diablo Field – gave the third highest muzzle energy, and third fastest muzzle velocity and, the important point, it also happens to give the best outdoor accuracy, and that’s the pellet I use.
It is commonly imagined that light pellets will be affected more by the wind than heavy ones, but the truth is a little more complicated than that.
A pellet’s susceptibility to wind drift is, in essence, a function of the length of time the pellet takes to reach the target, which in turn is a function of the pellet’s initial, or muzzle, velocity and how well it maintains velocity in flight, given by its ballistic coefficient (BC). For a given muzzle velocity, wind drift hence becomes a simple function of the BC.
In the UK, we tend to try to run our airguns to give the highest velocity that’s safely within the UK muzzle energy limit, usually a little over 11 ft. lb., with our chosen pellet, and so a formula to predict wind drift susceptibility at an equivalent muzzle energy would be extremely useful. Mike Wright came up with a formula he called the Wind Susceptibility Factor (WSF), which was the square root of the pellet mass in grains, divided by the pellet’s BC, which gave a three (sometimes two) digit number, and the lower the number, the less susceptible the pellet to wind drift.
With very limited facilities, we tested Mike’s equation and found that it was good at predicting which of two pellets would suffer less from wind drift, and gave a fairly accurate idea of by how much, at the same muzzle energy. The WSF was incorporated into Hawke Chairgun Pro, which lists WSF values alongside BC values in the pellet selection tables.
To promote the sale of spring airguns in countries with no legal energy limit, some manufacturers vie to be able to claim the highest muzzle velocity figures, which are achieved in part with big swept volume, long stroke and hefty pistons and springs, and in part by using ultralight non-lead pellets.
The metals in the alloys used to make non-lead pellets; nickel, tin and zinc, are much harder than lead, despite being annealed during manufacture to increase ductility, and so they will not seal at low air pressure, reducing the cylinder pulse. Once they have started to move, though, their very light mass (around 5 grains) ensures rapid acceleration, making the most of what cylinder pulse there is.
The high muzzle velocity might sound attractive, but the low sectional density of non-lead pellets means their velocity retention in flight will be markedly less than it would were they made of lead, possibly nearer that of a flat-head lead pellet than a domed-head lead pellet, so not only will they lose velocity rapidly, but they will be more susceptible to wind drift after they’ve lost that extra velocity.
Light pellets tend to cause slightly more spring airgun recoil, with less surge,
Light pellets can give higher muzzle velocity and energy in spring airguns, depending on pellet hardness.
Light pellets can give higher velocity but lower energy in PCPs, depending on pellet hardness.
Light pellets are not ‘blown around’ by the wind, as some suppose.
Above: In pellet selection, accuracy trumps flatter trajectory and better impact force.
Above: At just 13.4gn and with a low start pressure, the .22 Falcon Accuracy Plus could suit older, shorter-stroke spring airguns.
Above: The lightweight Falcon Accuracy Plus generates the least piston bounce of all the pellets I’ve tested.
Above: I shot .22 11.9gn RWS Hobby and 13.4gn Falcon Accuracy Plus into modelling clay to see which of these lightweights penetrated more.
Above: Even a cursory glance reveals that the Hobby penetrated less, in fact only two-thirds as far.
Above: The Hobby (left) hits with 1.5 times the impact force.
Above: The lightest pellet does not automatically produce the highest muzzle velocity from a PCP.
Above: PCP muzzle energy depends on far more than just pellet weight.