Jim Tyler explains how lost volume affects muzzle energy
The term ‘lost volume’ is used to describe the difference between the total combined volume of the cylinder, transfer port, pellet skirt and piston seal face, and the displacement of the piston stroke. The volume that is not lost volume is compressed, and is known as the ‘swept volume’.
What lost volume does is lower the cylinder pressure and energy for any given point in the piston’s compression stroke, so there is less energy to accelerate the pellet. At a perhaps more fundamental level, lowering the cylinder pressure retards the point in the compression stroke that the pellet starts to move, reducing the ‘cylinder pulse’; the length of piston travel in which the piston and the pellet are moving in the same direction, which means that the pellet accelerates for a shorter period, to a lower velocity.
BAD OR NOT?
So lost volume is a bad thing? Well, not necessarily. Most spring airguns have swept volume to spare, because they are designed with one eye on unrestricted markets with no ceiling on muzzle energy, such as North America, where high muzzle energy is a strong selling point. To accelerate a pellet to a high velocity takes a higher swept volume than is needed to accelerate a pellet to deliver under 12 ft.lbs., raising the question of whether increasing lost volume could be a viable way to reduce muzzle energy where desired, and it’s not just us in the UK who might wish to set our air rifles at less than their design maximum energy.
A recent trend amongst enthusiasts in the USA is reducing the muzzle energy of ‘magnum’ spring airguns to nearer or even within our 12 ft.lbs. limit or below, and the reason our American friends are doing this is that they believe it makes their rifles much more pleasant to shoot, and probably makes it easier to achieve desired accuracy.
In order to witness the effect of airgun modifications such as introducing lost volume, it is sometimes necessary to take them to extremes which, in the case of lost volume, I have already done.
GOING TO EXTREMES
A couple of years ago, I deliberately introduced massive lost volume into my old HW77 by fitting a range of short cylinder inserts with axial holes inside the compression tube, which
abruptly halted the piston part way through its usual stroke as it hit the insert, and fixed the peak cylinder pressure. By varying the length of the inserts, or altering their internal diameter, I was able to set peak cylinder pressure to practically any level I wanted, so I tested each set-up with a range of pellets to see which started to move, and which remained in the breech, to find each pellet’s start pressure.
One of the interesting findings from the experiment was that lightweight pellets with low start pressures were not only starting to move, but exiting the muzzle with peak air pressure at between 150-200psi, when the peak pressure was only a little higher than their start pressures, which ranged between 120psi (Falcon Accuracy Plus) and 150psi (Air Arms Express). The thought struck me that there must be a certain much lesser level of lost volume that would allow the muzzle energy for such pellets to be viable. Before considering whether tweaking the lost volume could be a viable way to regulate maximum muzzle energy, let’s consider the alternatives.
SETTING MUZZLE ENERGY
To reduce the muzzle energy of spring airguns that were designed to deliver muzzle energies in excess of our 12 ft.lbs. limit, there is a range of options. The obvious way is to fit a softer and perhaps less pre-loaded spring, which has the drawback of a more sluggish shot cycle, and necessitates finding a different mainspring. It is not easy to find a spring that is exactly what you want, because suppliers of aftermarket springs do not want to manufacture a spring that’s optimal for every airgun in every calibre, and so make up a small range of springs, and list the airguns that those springs will physically fit – sometimes they get it wrong, and a spring has to be cropped just to get it in the gun. Even when the springs fit with no alteration, the chance of them being anywhere near optimal for that airgun are small.
An alternative is to reduce the transfer port diameter, which on its own will increase piston bounce. The problem there is that increased piston bounce increases forward recoil surge, which starts whilst the pellet is still in the barrel, and is accelerating as the pellet exits the barrel. This can increase hold sensitivity, and make it more difficult to achieve accuracy, and it degrades the sight picture, making following through more of a challenge.
Another way to reduce muzzle energy is to reduce piston mass, which on its own will also increase piston bounce, with the same drawbacks as reducing the transfer port diameter. People do reduce piston mass because it reduces primary recoil but, for me, the jury is out on whether the accompanying increase in surge is too great a price to pay.
The currently fashionable way to reduce muzzle energy is to reduce the swept volume, which can be achieved in two ways; shortening the piston stroke, or sleeving the cylinder and fitting a narrower, lighter piston. Both reduce primary recoil, which is why they are popular.
I have shortened piston strokes by fitting longer piston rods, and by making piston extensions, and shortening the stroke was what I was effectively doing with the cylinder inserts in my pellet start pressure tests, so what if I shortened the stroke by fitting a cylinder insert, but with a much smaller hole than in the pellet start pressure tests, to give much higher peak air pressure? Could that offer a simpler, cheaper, way to reduce the piston stroke? Only one way to find out.
DYNAMIC LOST VOLUME
The pellet starts to move before piston bounce, and as it progresses up the barrel, the increasing volume of the barrel behind the pellet is effectively dynamic lost volume, which
makes it very difficult to predict what the peak cylinder pressure will be, because we don’t know how far up the barrel the pellet will be at piston bounce and, as my recent experiment with my TX200 and TX200 HC proved, each pellet accelerates at a different rate. In the absence of barrel lost volume data, we cannot calculate the effects of varying cylinder lost volume, so the only way to find out what the effects are is to test it and see what happens.
I decided to use my TX200 for the lost volume test, partly because it is such an easy rifle to strip and rebuild, partly because its central transfer port would maintain alignment with the hole in the insert even if the latter rotated.
I made the inserts from the 25mm diameter 90A SHORE polyurethane rod I have used to make piston seals. This is a soft material to machine, but since machining an insert consists of drilling a central hole and parting off to length, it’s not in the least bit difficult, and the cost of each insert is around 50p. I could have used a harder plastic such as Delrin, but the soft polyurethane held the promise of a cushioned piston landing at the end of the second forward stroke.
I started off with an insert 11mm long, to give a piston stroke of 85mm, and with a 10mm diameter hole, giving 864 cu mm of lost volume, equivalent to the barrel volume when a .177 pellet has travelled a couple of inches, and this gave 640fps with 8.4 grain pellets. After the first couple of test shots, I was aware of a tingling sensation in the pad of my trigger finger, which told me that the piston was colliding with the insert at some speed, pushing the whole rifle violently forward. Due to the combination of piston ‘slam’ and low muzzle energy, I terminated the test without bothering to record the recoil cycle.
The next insert was 10mm long, with a 6mm axial hole, giving a much reduced 282 cu mm of lost volume. With 7.87 grain pellets, and the same standard Air Arms spring and 42mm preload as I use in my conventionally (longer piston rod) short-stroked TX200, this gave velocities a little under 790fps – just under 11 ft.lbs. – close to what I get with my conventionally short-stroked TX200.
I decided to reduce the lost volume by fitting a 10mm long insert with an 3.8mm hole, giving 113 cu mm of lost volume or, thought of in another way, effectively a 19.8mm long transfer port, which will result in a slight loss of energy efficiency, having said which, after a few shots to settle the action, the velocity had increased to 800fps, for 11.1 ft.lbs. That’s just 10fps down from what I get with the ‘conventionally’ short-stroked rifle – partly due to lesser energy efficiency, partly
due to the lower piston weight – at a fraction of the expense and trouble.
To short stroke using a longer piston rod, you have to buy the rod, then remove the piston and seal, heat the piston to loosen the thread locking compound, unscrew the rod, run a tap through the threaded hole in the piston to clean it, make a tool, for which you need a lathe, to keep the new rod central when you refit it, apply thread lock compound, screw in the new rod, and leave it for the thread lock compound to harden.
To short stroke using a cylinder insert, you remove the piston, push the insert up the cylinder – it will be a tight fit – and put the rifle back together. Job done.
THE SHOT CYCLE
The lost volume in the 3.8mm cylinder insert bore retards the point in the compression stroke that the 7.87 grain pellet starts to move, which in this case is around 70mm into the stroke, but by no more than the tiniest fraction of a millimetre, lowering the air pressure at the 70mm, 150psi mark by just 2 psi, so that has no measurable effect on muzzle velocity.
In use, the shot cycle of the rifle with the insert feels indistinguishable from that of the same rifle with a lengthened piston rod, with the total time to pellet exit, and piston velocities being virtually identical. In a blind test, I very much doubt that anyone could discern whether the stroke was shortened using a longer rod, a piston extension, or a 50p polyurethane cylinder insert.
LESS IS MORE
In fact, when I measured the shot cycle, the rifle with the cylinder insert recoiled less, and had less surge than the rifle with the same piston stroke and an extended piston rod or a piston extension. The lower recoil I can understand, because the shorter piston rod weighs less, but the lower surge is less easily explained; all I can think is that more of the compressed air is following the pellet out of the muzzle instead of driving piston bounce.
I am not commending this modification just yet, because at the time of writing. it has only been tested with a couple of hundred pellets, and it really needs in the region of 10,000 shot cycles to show up any snags. All looking good so far, though.
With most production spring airguns, a modest amount of lost volume seems not in itself a huge problem, because it represents such a small percentage of the swept volume. With spring airguns that have been modified to reduce swept volume to a bare minimum, as well as airguns with low swept volumes to begin with, even modest lost volume will undoubtedly be undesirable.
The rifle is very well behaved with its polyurethane insert.
The rifle needs the same preload with an insert as it does with a longer rod or piston extension.
It’s quite easy to cross thread the end cap, so fit it by hand, and carefully.
Make sure that the end plug is screwed fully home; even a small gap will cause problems.
When machining soft polyurethane, use sharp tools and low speeds.
The end plug is fitted with thread lock compound, which has to be softened by applying heat here.
I used a mini-drill and stone to put a slight taper into the insert, which may or may not aid air flow.
Three methods of short-stroking. The cylinder insert is by far the cheapest, and seems just as good.
To fit a piston rod, you need a tool to keep it centred whilst the thread lock compound cures.
I tried different lengths and various axial hole sizes.
My pellet start pressure experiments proved that massive lost volume was disastrous to muzzle energy, and caused piston slam.
With the scope back on, the rifle proved to be at least as accurate with the cylinder insert as with an extended piston rod.