SHORT-STROKE UP­DATE

How is Jim’s ul­tra-low­cost, short-stroked TX200 stand­ing up to the rigours of use?

Airgun World - - Technical Airgun -

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.

TX200 HC

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.

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