How is Jim’s ultra-lowcost, short-stroked TX200 standing up to the rigours of use?
For the benefit of anyone who missed the last two episodes, I decided to carry out experiments into the effects of lost volume in the spring airgun by fitting short cylindrical inserts in the front end of the cylinder of my TX200, which reduced the available piston stroke and introduced lost volume. In the course of these experiments, it occurred to me that fitting short inserts in the region of 10mm long might prove a viable way of short-stroking the Mk.3 TX for a matter of pennies. It worked.
I ended up with a 10mm long insert made from 90A SHORE polyurethane, which is a soft material with the potential to cushion piston landing, with a 3.6mm axial hole, which shortened the 96mm stroke to 86mm, and effectively gave a 3.6mm diameter by 19.8mm long transfer port. The resultant shot cycle was practically indistinguishable from that of the rifle when short-stroked via a more expensive lengthened piston rod or piston extension.
Next, I made an insert with the same dimensions out of Acetal, which machines a lot more easily than soft polyurethane, with an ‘O’ ring to provide a seal, and this, worked a treat, too – so well, in fact, that it warranted extensive testing. Incidentally, the reason I especially want to conduct a long-term test for this insert is that Acetal has a melting point of 170C, which is exceeded for around a millisecond or two each shot, and I need to ensure that ten thousand 1 - 2 millisecond exposures to circa 600C air will not cause cumulative damage by melting and enlarging. or deforming the hole in the insert.
The modified TX has now shot just over one tin of pellets, and is performing beautifully, not only in returning 11.6ft.lb. with 7.87gn Air Arms Express pellets, but also grouping effortlessly from the bench on the 40-yard range. The 500 pellets used so far constitute only 5% of the total that the rifle needs to be tested with, though, so unless I can motivate myself for some serious, soul-destroying testing, the final results will be some months off.
By way of a diversion, and to gauge the longer term robustness of polyurethane cylinder inserts, I fitted my TX200 HC with an 8mm-long polyurethane insert, reducing the stroke to 88mm, the same as a Walther LGU. I gave the HC a couple of millimetres extra stroke over the full-length rifle in the hope of compensating for the HC’s shorter barrel, which is less energy efficient. Using a standard Air Arms mainspring, I set the HC preload at 39mm for a target muzzle energy of 11ft.lb., which proved to be bang on with .177 Air Arms Express.
One slight glitch that was immediately apparent on loading the rifle was that the second anti-bear trap notches – there are three on the Mk.3 – and trigger sear were engaging simultaneously, so that the trigger mechanism and anti-bear trap were sharing the pressure from the compressed mainspring, and to release the anti-bear trap mechanism, it is necessary to take the weight of the spring by pulling the underlever. With the TX, this is a minor annoyance, but with the Pro Sport it would be a serious problem because the anti-bear trap mechanism is concealed under the stock, so the rifle is cocked, cannot be fired, and can only be de-cocked with the action out of the stock
which, take it from me, is a tricky and potentially hazardous operation. For that reason, I strongly suggest NOT trying DIY stroke modifications with the Pro Sport unless working with a known ‘safe’ stroke.
If I have not made any mistakes in my measurements and calculations, then thanks to the short HC barrel, pellet exit appears to be two-thirds of a millisecond after piston bounce, at which point, the rifle has surged by just one twentieth of a millimetre. That bodes well for ‘field’ (as opposed to bench) accuracy, so the rifle has promise as a competitive HFT springer.
TX200 OR HC?
The dilemma faced by TX200 buyers is whether to opt for the full-length rifle or the shorter-barrel HC and as I have been shooting both for some time, these are my thoughts, for what they’re worth. With identical power plants, the full-length barrel of the TX200 delivered between 0.4 ft. lb. and 0.6 ft. lb. more, depending on stroke, in my tests, so that for equivalent muzzle energy, the full rifle would have the better shot cycle, as well as lower cocking effort.
For equivalent muzzle energy, the HC gives roughly 9% more recoil travel and 30% more surge travel, although most of the extra surge occurs after the pellet has exited the muzzle, so the difference in surge is in sight picture disturbance during the pellet’s flight, which has no direct bearing on accuracy, but makes the full-length rifle feel a little better behaved. The extra weight of the full rifle and especially the extra weight’s position at the muzzle also helps to improve the recoil cycle.
All in all, for any target discipline, as well as static hunting where the rifle can be supported, I’d favour the full rifle. However, I have found the HC better for me when shot standing using the sporting rifle hold. This is mainly due to the HC having slightly less weight up front, possibly aided by the marginally quicker shot cycle, so for rough shooting, the HC edges it.
Having said all that, either variant will fulfil any role admirably, and the choice between the two comes down to a matter of personal preference, with the best option of owning both.
The TX200 HC and Niko 3-9x40mm scope are a good all-round combination.
The HC was previously short-stroked with a screw-on piston extension I made from aluminium. The insert works just as well, if not better.
The HC exits the pellet 0.59 milliseconds ahead of the standard TX200.
The calculated recoil of the full TX with a 0.5 kg scope, shooting Express pellets at 814fps.
The calculated recoil of the TX HC with a 0.5 kg scope shooting Express pellets at 798fps.
Polyurethane is not the easiest of materials to machine, but the machining involved in making cylinder inserts is not difficult.