Lost Volume 2
Jim follows up his TX200 budget short-stroke experiment and tests an even more economical, and seemingly better, alternative
Jim expands on his muzzle energy dilemma, and finds an even cheaper way to solve it
One of the most appealing aspects of spring-airgun experimental engineering is that you never know exactly where an experiment might lead, nor which new insights into the workings of the gun you might stumble upon. So it was, that an experiment into the effects of creating lost volume led to a seemingly viable way of short-stroking airguns at a fraction of the usual cost, as covered in last month’s article.
I shared my findings ahead of publication with my friend David Robson who, like me, is fascinated by the spring airgun and devotes a lot of time to devising ways of measuring the shot cycle, but who, unlike me, is a retired chartered engineer with a university education in science, and he was keen to join in the tests, but I had run out of the polyurethane rod I’d used in my experiments, so the search was on for an alternative.
One material we both have in abundance is Acetal, which we use to make spring guides and top hats, and I suggested making cylinder inserts from this, with an ‘O’ ring to provide both a seal and to hold the insert at the front of the cylinder. I was quicker to the draw, and was able to report to David that the Acetal insert was giving me a gain of 13fps to 20fps with Air Arms Express over the polyurethane insert. Now the testing could begin.
HOW IT’S MADE
I turned a length of Acetal rod down to an easy sliding fit in the cylinder, then drilled an axial 3.7mm hole and chamfered the inlet. Next, I ground the end of a thin parting-off tool into a ‘U’ shape, and used it to cut a groove for the ‘O’ ring, which I trial fitted, increased the depth of the hole, refitted until it could enter the cylinder, then parted off the insert at 10mm.
The ‘O’ ring serves two purposes; first, it holds the insert at the front of the cylinder, and second, it seals the small amount of lost volume surrounding the insert. The Acetal rod costs around £10 for a one-metre length, so a
10mm section costs 10p, and a 25mm ‘O’ ring adds a few pence to that, so this is probably the cheapest airgun modification there is, or is ever likely to be.
I was determined to use the standard Mk.3 TX200 spring, which is top quality, has 23 active coils, an outside diameter of 20.8mm, and is wound from 2.9mm wire, for a stiffness of 5.38 N/mm (Newtons per millimetre), which is fairly soft compared to most aftermarket alternatives.
Soft springs work rather differently from stiff springs, accelerating the piston more gently for a slightly shorter distance, but using preload force to keep the piston in the vicinity of the cylinder end for longer. This means that softer springs create slightly lower peak pressure and temperature, but maintain them for longer. Some people, including my engineer friend David Robson, prefer shorter, stiffer springs, which give a more rapid compression stroke for a feel most commonly described as ‘quick’, which give greater piston bounce due to less preload force, but give a softer eventual piston landing for the same reason.
Which is ‘right’; short and stiff or long and soft? The answer is that, within bounds of reason, neither is right, nor wrong; they’re different, just as David and I are different in our preferred shot cycle.
The main reason for using the standard spring in this case was not shot-cycle preference, but economy. If you’re playing with a conversion that costs a few pennies, why spend quite a lot of pounds on a replacement mainspring?
With 42mm of spring preload, the rifle achieved 11.4 ft. lb. with 7.87 grain Air Arms Express, 10.9 ft.lbs. with Air Arms Field. Increasing preload to 43mm increased that to 11.6 ft. lb. with the lighter pellet, 11.3 ft.lbs. with the heavier, perfect for the intended purpose of HFT.
The recoil is actually fractionally less than you’d get for the same piston stroke with a long piston rod or piston extension, because the standard piston weighs a few grams less and, with a 500 gram scope and mounts fitted, recoil is just over 4.5mm, followed by under 0.75mm of surge, with the Air Arms Express pellet exiting the muzzle after less than one tenth of a millimetre of forward surge, and around 0.7 milliseconds after piston bounce, all of which has to be good for accuracy.
The measurements above were recorded with the rifle fairly free to recoil, sliding in a cradle; the recoil and surge when the rifle is in normal use will vary according to the way in which it is supported and restrained.
I proved some years ago that there was a link between transfer port length and energy efficiency, with longer ports tending to be less energy efficient, and increasing the transfer port of the TX200 from 9.8mm to 19.8mm will have reduced efficiency. The usual consequence of decreasing efficiency is an increase in piston bounce (and hence recoil surge) as energy that should push the pellet pushes the piston back up the cylinder instead, but in this case, that seems not to happen, suggesting that the lost energy simply follows the pellet up the barrel to vent to atmosphere.
With the 10mm insert, the rifle needs two millimetres more spring preload than it does with an extended piston rod, which raises cocking effort, but by so little that you won’t be aware of it, and it does increase the piston landing velocity at the end of the second forward stroke, but only to 3M/s, which again, you won’t notice.
The lessened energy efficiency with the longer transfer port is not all due to the port length itself, because there will be a small drop in muzzle energy due to the fact that the piston is lighter by 6 grams than one with a longer piston rod, and more with a piston extension, and I suspect the energy loss attributable to the longer port is fairly low.
With 42mm of spring preload, the shot cycle excited the spring’s natural resonance, resulting in an annoying 126Hz twang following piston landing. It wasn’t loud as spring twangs go, and could only be heard by the person shooting and with their ear against the stock, but it is a distraction when shooting, and the mainspring throwing its mass back and forth was causing the rifle to oscillate rapidly back and forth. A little grease on the mainspring initially sorted it, but when I increased preload to 43mm, the twang disappeared of its own accord.
Let’s start by acknowledging that the TX200 in standard form is capable of stacking quality pellets one on top of another at normal shooting range (the user might not always be capable, but the rifle is), and that any gain in real-world accuracy from a modification has to be a result of the rifle being easier to shoot.
It’s a very subjective thing, but I find the modified rifle easier to shoot, certainly on a par with, if not better than, the rifle when short-stroked using an extended piston rod or piston extension, and I’m not alone in that, because everyone who has shot the rifle has commented favourably on both the feel and accuracy.
The cylinder insert lends itself to airguns with central transfer ports like the TX200 and LGU, but in airguns with offset ports, the worry would be the insert moving and losing alignment with the transfer port. In practice, the insert would probably only move in rifles with pistons that are free to turn under the torque of the expanding mainspring, such as the Walther LGV, but in airguns that have a slotted piston that engages a cocking shoe, the piston cannot rotate and the insert would probably be OK.
Mentioning the LGV raises an issue with rifles that already have long transfer ports (the LGV’s is 26.9mm), because lengthening them would take you into uncharted territory in terms of fluid dynamics and thermodynamics; my gut feeling is that such long ports would cost dearly in muzzle energy but, in the absence of hard test data, nobody can tell for certain.
Any spring airgun modification should be tested thoroughly, because a few shots, or even a few tins of pellets, will not show up any long-term problems, and I will be looking to put around 10,000 pellets through the rifle, and to test the modification in other actions, before I would be happy to recommend it. I would most certainly not recommend trying the modification on a hunting or competition rifle just yet!
Testing revealed the shot cycle to be practically indistinguishable from rifles with longer piston rods or piston extensions.
The first step was to turn the Acetal rod so that it was an easy sliding fit in the cylinder. I used a parting-off tool with the cutting end rounded to cut the ‘O’ ring groove.
I had to deepen the ‘O’ ring groove until it could slide into the cylinder.
The shot cycle with Air Arms Express is excellent, with pellet exit just after piston bounce.
11.3 ft. lb. with Air Arms Express pellets, perfect for HFT.
The insert fits with the ‘O’ ring at the piston end to minimise lost volume.
I set the length of the insert at 10mm ready for parting off.