Lost Vol­ume 2

Jim fol­lows up his TX200 bud­get short-stroke ex­per­i­ment and tests an even more eco­nom­i­cal, and seem­ingly bet­ter, al­ter­na­tive

Airgun World - - Contents -

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.


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.

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