Jim Tyler helps to soft-tune a Walther LGV to achieve an ‘out of this world’ standard of accuracy
Iremember reading the editor’s review of the Walther LGV which was, as far as I’m aware, the first published review, and being so surprised at the lavish praise Terry heaped upon the rifle that I called him and asked whether it really was that good. There was no disguising the enthusiasm in his voice, and I knew that the rifle really was as good as he’d said.
There were a few naysayers on the Internet, of course, people who had never shot the rifle and yet knew it wasn’t as good as Terry had indicated, but Terry took the test rifle to a number of shows, and the sometimes vociferous naysayers who tested it on the range were strangely subdued afterwards. Then it was my turn.
I tested the rifle’s accuracy; stripped it down, weighed and measured everything, and published my findings, including the dimensions of the transfer port which, at 26.9mm x 2.6mm, seemed to run against commonly accepted wisdom that transfer ports should be as short as possible. I freely hold my hands up to discovering a relationship between port length and energy efficiency the year before when testing my HW77, and publishing it, and it seems a lot of people had taken that as gospel, judging by the reaction on the Internet to the LGV’s transfer port dimensions.
It wasn’t just the transfer port’s length that people seized upon; it was also the 2.6mm diameter, which was widely thought to restrict the flow of air, reducing the muzzle energy, and some went as far as to speculate that this placed huge strain on the mainspring, damaging it. The truth is that we don’t know enough about the physics of the spring airgun (most especially the little researched field of compressible fluid dynamics) to reach such conclusions, which I believe are both questionable.
Possibly due to people thinking the transfer port length was preventing the rifle reaching its full potential, the tuning community don’t seem to have spent much time working with the LGV, possibly concentrating of the under lever LGU, HW97 and TX200 instead. That’s a shame, because the LGV is a rifle with great potential, despite the long transfer port.
After publishing my test findings on the LGV, I went back to working with my then two main experimental rifles, the HW77 and HW95, and the LGV took very much a back seat for some years, until very recently. A member of the excellent Shooting the Breeze (STB) web site (https://shooting-the-breeze.com/) contacted me with a question about altering the transfer port diameter of the LGV, but – me being me
– I read it as a question about the LGU, which generates far greater interest from the tuning perspective, and answered the question, adding my thoughts on a suitable alternative mainspring, based on recent experiments with my LGU. It was not until the member came back to me that I read the message properly, and realised he had an LGV, had fitted the spring I’d recommended, and reported that the result was ‘out of this world’. I did the maths, and the spring I had recommended had made 29 ft.lbs. available to the piston, which gives a muzzle energy of 11.6 ft.lbs., for an efficiency of 39%, which is good for a break-barrel, and rather suggests that the long and thin transfer port is not significantly stifling the muzzle energy. The spring in question is probably the best off-the-shelf mainspring in terms of quality and longevity that I’m aware of, and it’s the factory standard Mk.3 Air Arms TX200 spring that works well in my LGU, and which is a very different beast from the standard Walther spring. Both springs are 20.8mm external diameter, but that’s just about all they have in common; the TX spring has just 23 coils against the Walther’s 32, the TX wire is 2.9mm against the Walther’s 3.25mm, and the TX spring rate is 5.36 N/mm against the Walther’s 6.39 N/mm.
The standard Walther spring when new has around 26mm of preload, and the trick when
“in this case it was the heavier pellet that produced the better accuracy”
switching to a softer spring is to increase preload, which will have a fundamental effect on the shot cycle, and even how the rifle manages to achieve its muzzle energy.
The stiffer spring makes more energy available to the piston, so the piston stroke increases slightly, increasing the peak cylinder pressure and temperature, but once the piston comes to a halt, the greater pressure from the compressed air remaining in the cylinder is met with less opposing force from the less preloaded mainspring, so the piston bounces faster and further. The softer spring makes less energy available to the piston, so the piston travels a fraction less distance so that when it comes to a halt, there is lower air pressure to drive bounce against a higher opposing force from the greater preload, so the piston bounces more slowly and less. The shot cycles have a fundamental effect of felt recoil.
Most people shooting a spring airgun fitted with a stiff spring remark that it feels ‘quick’, but whether they are sensing a difference between, say, nine and a half and 10 milliseconds, or whether they are sensing fractionally greater recoil travel or, and most likely, they are sensing faster recoil acceleration, is open to question. I very much suspect it’s the latter. In contrast, a soft, heavily preloaded spring gives a gentle recoil acceleration, so some will observe that it feels ‘slow’.
The TX200 spring might be the same outside diameter as the Walther spring, but it has thinner wire, so its internal diameter is greater, which makes it a slack fit on the LGV’s spring guide. The spring is thus fairly free to vibrate, which is easily dampened with grease, but there’s another issue; the spring can buckle just enough during the latter stage of the cocking stroke to contact the piston wall which, not being very smooth, makes the cocking stroke feel slightly rough. The solution is to make, or have made, a correctly sized spring guide and top hat, which I will get around to in the fullness of time.
With 40mm of preload, the rifle was far too close to the legal limit with Air Arms Field and Express, so I reduced the preload by degrees, stopping at 36mm, which gave 11.3 ft.lbs. at the muzzle, which with Air Arms Field meant a muzzle velocity a few fps either side of 778fps, pretty much perfect for HFT, for which the LGV is probably the best suited break-barrel, fully capable of mixing it with the heavy under-levers that dominate the HFT Recoil class.
My test rig measured recoil at just under 5mm, which would reduce to less than 4.5mm with a 0.5kg scope, with pellet exit (AA Field) occurring just 0.2mm into the total of 1.04mm of surge. The time of the entire shot cycle to pellet exit was 11.22 milliseconds (ms), the compression stroke took 9.9ms with an average piston velocity of 8.9 metres per second (M/s), peak piston velocity of 14.95 M/s, and piston landing of just under 4 M/s. The piston landing velocity is rather higher than I expected, possibly a side effect of the transfer port diameter causing mass air flow to choke a little too early in the stroke. With 7.8g Air Arms Express pellets, the piston landing velocity was more in line with expectations at 3.1 M/s.
After the piston lands, any further rifle movement/vibration can only be caused by the mainspring throwing its weight around, and in this respect the shot cycle was rather more refined with 7.8g pellets. With the 8.4g (AA Field) pellets, minor vibration continued up to the point at which the pellet will have travelled approximately 25 yards, but with the 7.8g (AA Express) pellets, all movement and vibration ceased when the pellet had travelled just five yards which, in theory, has to be good for accuracy. Theory and practice don’t always agree, though, and in this case it was the heavier pellet that produced the better accuracy.
Mike Wright concluded that altering the diameter of the transfer port should be the last stage in tuning a spring airgun, and that’s my
“it boils down to using gut feeling to decide which seems most promising”
approach. The main effect of altering the transfer port is not, as seems logical, to allow more air through the bigger hole to propel the pellet, but is to alter the point in the shot cycle at which the mass air flow chokes and with this particular LGV tune, I want to delay choking to better suit the 8.4g pellets, which will mean enlarging the port.
What size to make the port, though? I do have three formulas based on matching the cylinder and barrel pulses, matching the piston and pellet velocity, or taking the pulse as a percentage of the stroke – all at the point of piston bounce, but although some of my recorded measurements are used, all three formulae are based to a degree on a number of educated guesses, such as how far the pellet is up the barrel at piston bounce, so this is not an exact science. The three formulas always produce three different results, anyway, and in the end it boils down to using gut feeling to decide which seems most promising.
Matching the velocities of the piston and pellet at piston bounce gave a suggested port diameter of 2.9mm; matching the pulses suggested 3.26mm, and taking the cylinder pulse as a percentage of the stroke suggested 3.1mm. The formulas take no account of transfer port length and with the LGV transfer port being very long and hence having a large volume, I’ll probably opt for a conservative 2.9mm. That’s for the future, though, for two reasons; first, I don’t have a drill bit suitable at the present, and second, the rifle needs more thorough testing as it is before altering anything else, which will entail a lot of range work, and some HFT competition.
If you want to try the shot cycle from a softer spring in your LGV, the Mk.3 TX200 spring fits the bill nicely, although you will need preload washers and, to get the best out of it, new spring guides.I
A soft spring and a long barrel give low cocking effort.
My recoil measuring set-up suggests this LGV tune holds much promise.
My low-tech solution to operating the trigger without moving the rifle when recoil testing.
The manual barrel lock is rock solid.
With 36mm of preload, it takes little force (42 lbf) to fit the spring.
The shot cycle is gentle with Air Arms Field, and the accuracy appears second to none amongst sporting spring airguns.
With Air Arms Express, surge is less than that with the heavier Field.
The digital vernier caliper is invaluable for measuring things like preload.
Looks like a piece of grit found its way under the cocking arm bearing at some point. That needs polishing out.
The captive spring guide is about half a millimetre less than ideal in diameter for the TX200 spring.
The LGV piston seal works well enough; some people fit aftermarket seals, which can improve energy efficiency a tad.