Lost Vol­ume

Jim Tyler ex­plains how lost vol­ume af­fects muz­zle en­ergy

Airgun World - - Contents -

The term ‘lost vol­ume’ is used to de­scribe the dif­fer­ence be­tween the to­tal com­bined vol­ume of the cylin­der, trans­fer port, pel­let skirt and pis­ton seal face, and the dis­place­ment of the pis­ton stroke. The vol­ume that is not lost vol­ume is com­pressed, and is known as the ‘swept vol­ume’.

What lost vol­ume does is lower the cylin­der pres­sure and en­ergy for any given point in the pis­ton’s com­pres­sion stroke, so there is less en­ergy to ac­cel­er­ate the pel­let. At a per­haps more fun­da­men­tal level, low­er­ing the cylin­der pres­sure re­tards the point in the com­pres­sion stroke that the pel­let starts to move, re­duc­ing the ‘cylin­der pulse’; the length of pis­ton travel in which the pis­ton and the pel­let are mov­ing in the same di­rec­tion, which means that the pel­let ac­cel­er­ates for a shorter pe­riod, to a lower ve­loc­ity.

BAD OR NOT?

So lost vol­ume is a bad thing? Well, not nec­es­sar­ily. Most spring air­guns have swept vol­ume to spare, be­cause they are de­signed with one eye on un­re­stricted mar­kets with no ceil­ing on muz­zle en­ergy, such as North Amer­ica, where high muz­zle en­ergy is a strong sell­ing point. To ac­cel­er­ate a pel­let to a high ve­loc­ity takes a higher swept vol­ume than is needed to ac­cel­er­ate a pel­let to de­liver un­der 12 ft.lbs., rais­ing the ques­tion of whether in­creas­ing lost vol­ume could be a vi­able way to re­duce muz­zle en­ergy where de­sired, and it’s not just us in the UK who might wish to set our air ri­fles at less than their de­sign max­i­mum en­ergy.

A re­cent trend amongst en­thu­si­asts in the USA is re­duc­ing the muz­zle en­ergy of ‘mag­num’ spring air­guns to nearer or even within our 12 ft.lbs. limit or be­low, and the rea­son our Amer­i­can friends are do­ing this is that they be­lieve it makes their ri­fles much more pleas­ant to shoot, and prob­a­bly makes it eas­ier to achieve de­sired ac­cu­racy.

In or­der to wit­ness the ef­fect of air­gun mod­i­fi­ca­tions such as in­tro­duc­ing lost vol­ume, it is some­times nec­es­sary to take them to ex­tremes which, in the case of lost vol­ume, I have al­ready done.

GO­ING TO EX­TREMES

A cou­ple of years ago, I de­lib­er­ately in­tro­duced mas­sive lost vol­ume into my old HW77 by fit­ting a range of short cylin­der in­serts with ax­ial holes in­side the com­pres­sion tube, which

abruptly halted the pis­ton part way through its usual stroke as it hit the in­sert, and fixed the peak cylin­der pres­sure. By vary­ing the length of the in­serts, or al­ter­ing their in­ter­nal di­am­e­ter, I was able to set peak cylin­der pres­sure to prac­ti­cally any level I wanted, so I tested each set-up with a range of pel­lets to see which started to move, and which re­mained in the breech, to find each pel­let’s start pres­sure.

One of the in­ter­est­ing find­ings from the ex­per­i­ment was that light­weight pel­lets with low start pres­sures were not only start­ing to move, but ex­it­ing the muz­zle with peak air pres­sure at be­tween 150-200psi, when the peak pres­sure was only a lit­tle higher than their start pres­sures, which ranged be­tween 120psi (Fal­con Ac­cu­racy Plus) and 150psi (Air Arms Ex­press). The thought struck me that there must be a cer­tain much lesser level of lost vol­ume that would al­low the muz­zle en­ergy for such pel­lets to be vi­able. Be­fore con­sid­er­ing whether tweak­ing the lost vol­ume could be a vi­able way to reg­u­late max­i­mum muz­zle en­ergy, let’s con­sider the al­ter­na­tives.

SET­TING MUZ­ZLE EN­ERGY

To re­duce the muz­zle en­ergy of spring air­guns that were de­signed to de­liver muz­zle en­er­gies in ex­cess of our 12 ft.lbs. limit, there is a range of op­tions. The ob­vi­ous way is to fit a softer and per­haps less pre-loaded spring, which has the draw­back of a more slug­gish shot cy­cle, and ne­ces­si­tates find­ing a dif­fer­ent main­spring. It is not easy to find a spring that is ex­actly what you want, be­cause sup­pli­ers of af­ter­mar­ket springs do not want to man­u­fac­ture a spring that’s op­ti­mal for ev­ery air­gun in ev­ery cal­i­bre, and so make up a small range of springs, and list the air­guns that those springs will phys­i­cally fit – some­times they get it wrong, and a spring has to be cropped just to get it in the gun. Even when the springs fit with no al­ter­ation, the chance of them be­ing any­where near op­ti­mal for that air­gun are small.

An al­ter­na­tive is to re­duce the trans­fer port di­am­e­ter, which on its own will in­crease pis­ton bounce. The prob­lem there is that in­creased pis­ton bounce in­creases for­ward re­coil surge, which starts whilst the pel­let is still in the bar­rel, and is ac­cel­er­at­ing as the pel­let ex­its the bar­rel. This can in­crease hold sen­si­tiv­ity, and make it more dif­fi­cult to achieve ac­cu­racy, and it de­grades the sight pic­ture, mak­ing fol­low­ing through more of a chal­lenge.

An­other way to re­duce muz­zle en­ergy is to re­duce pis­ton mass, which on its own will also in­crease pis­ton bounce, with the same draw­backs as re­duc­ing the trans­fer port di­am­e­ter. Peo­ple do re­duce pis­ton mass be­cause it re­duces pri­mary re­coil but, for me, the jury is out on whether the ac­com­pa­ny­ing in­crease in surge is too great a price to pay.

SWEPT VOL­UME

The cur­rently fash­ion­able way to re­duce muz­zle en­ergy is to re­duce the swept vol­ume, which can be achieved in two ways; short­en­ing the pis­ton stroke, or sleev­ing the cylin­der and fit­ting a nar­rower, lighter pis­ton. Both re­duce pri­mary re­coil, which is why they are pop­u­lar.

I have short­ened pis­ton strokes by fit­ting longer pis­ton rods, and by mak­ing pis­ton ex­ten­sions, and short­en­ing the stroke was what I was ef­fec­tively do­ing with the cylin­der in­serts in my pel­let start pres­sure tests, so what if I short­ened the stroke by fit­ting a cylin­der in­sert, but with a much smaller hole than in the pel­let start pres­sure tests, to give much higher peak air pres­sure? Could that of­fer a sim­pler, cheaper, way to re­duce the pis­ton stroke? Only one way to find out.

DY­NAMIC LOST VOL­UME

The pel­let starts to move be­fore pis­ton bounce, and as it pro­gresses up the bar­rel, the in­creas­ing vol­ume of the bar­rel be­hind the pel­let is ef­fec­tively dy­namic lost vol­ume, which

makes it very dif­fi­cult to pre­dict what the peak cylin­der pres­sure will be, be­cause we don’t know how far up the bar­rel the pel­let will be at pis­ton bounce and, as my re­cent ex­per­i­ment with my TX200 and TX200 HC proved, each pel­let ac­cel­er­ates at a dif­fer­ent rate. In the ab­sence of bar­rel lost vol­ume data, we can­not cal­cu­late the ef­fects of vary­ing cylin­der lost vol­ume, so the only way to find out what the ef­fects are is to test it and see what hap­pens.

I de­cided to use my TX200 for the lost vol­ume test, partly be­cause it is such an easy ri­fle to strip and re­build, partly be­cause its cen­tral trans­fer port would main­tain align­ment with the hole in the in­sert even if the lat­ter ro­tated.

I made the in­serts from the 25mm di­am­e­ter 90A SHORE polyurethane rod I have used to make pis­ton seals. This is a soft ma­te­rial to ma­chine, but since ma­chin­ing an in­sert con­sists of drilling a cen­tral hole and part­ing off to length, it’s not in the least bit dif­fi­cult, and the cost of each in­sert is around 50p. I could have used a harder plas­tic such as Del­rin, but the soft polyurethane held the prom­ise of a cush­ioned pis­ton land­ing at the end of the sec­ond for­ward stroke.

DIF­FER­ENT STROKES

I started off with an in­sert 11mm long, to give a pis­ton stroke of 85mm, and with a 10mm di­am­e­ter hole, giv­ing 864 cu mm of lost vol­ume, equiv­a­lent to the bar­rel vol­ume when a .177 pel­let has trav­elled a cou­ple of inches, and this gave 640fps with 8.4 grain pel­lets. Af­ter the first cou­ple of test shots, I was aware of a tin­gling sen­sa­tion in the pad of my trig­ger fin­ger, which told me that the pis­ton was col­lid­ing with the in­sert at some speed, push­ing the whole ri­fle vi­o­lently for­ward. Due to the com­bi­na­tion of pis­ton ‘slam’ and low muz­zle en­ergy, I ter­mi­nated the test with­out both­er­ing to record the re­coil cy­cle.

The next in­sert was 10mm long, with a 6mm ax­ial hole, giv­ing a much re­duced 282 cu mm of lost vol­ume. With 7.87 grain pel­lets, and the same stan­dard Air Arms spring and 42mm preload as I use in my con­ven­tion­ally (longer pis­ton rod) short-stroked TX200, this gave ve­loc­i­ties a lit­tle un­der 790fps – just un­der 11 ft.lbs. – close to what I get with my con­ven­tion­ally short-stroked TX200.

I de­cided to re­duce the lost vol­ume by fit­ting a 10mm long in­sert with an 3.8mm hole, giv­ing 113 cu mm of lost vol­ume or, thought of in an­other way, ef­fec­tively a 19.8mm long trans­fer port, which will re­sult in a slight loss of en­ergy ef­fi­ciency, hav­ing said which, af­ter a few shots to set­tle the ac­tion, the ve­loc­ity had in­creased to 800fps, for 11.1 ft.lbs. That’s just 10fps down from what I get with the ‘con­ven­tion­ally’ short-stroked ri­fle – partly due to lesser en­ergy ef­fi­ciency, partly

due to the lower pis­ton weight – at a frac­tion of the ex­pense and trou­ble.

To short stroke us­ing a longer pis­ton rod, you have to buy the rod, then re­move the pis­ton and seal, heat the pis­ton to loosen the thread lock­ing com­pound, un­screw the rod, run a tap through the threaded hole in the pis­ton to clean it, make a tool, for which you need a lathe, to keep the new rod cen­tral when you re­fit it, ap­ply thread lock com­pound, screw in the new rod, and leave it for the thread lock com­pound to har­den.

To short stroke us­ing a cylin­der in­sert, you re­move the pis­ton, push the in­sert up the cylin­der – it will be a tight fit – and put the ri­fle back to­gether. Job done.

THE SHOT CY­CLE

The lost vol­ume in the 3.8mm cylin­der in­sert bore re­tards the point in the com­pres­sion stroke that the 7.87 grain pel­let starts to move, which in this case is around 70mm into the stroke, but by no more than the tini­est frac­tion of a mil­lime­tre, low­er­ing the air pres­sure at the 70mm, 150psi mark by just 2 psi, so that has no mea­sur­able ef­fect on muz­zle ve­loc­ity.

In use, the shot cy­cle of the ri­fle with the in­sert feels in­dis­tin­guish­able from that of the same ri­fle with a length­ened pis­ton rod, with the to­tal time to pel­let exit, and pis­ton ve­loc­i­ties be­ing vir­tu­ally iden­ti­cal. In a blind test, I very much doubt that any­one could dis­cern whether the stroke was short­ened us­ing a longer rod, a pis­ton ex­ten­sion, or a 50p polyurethane cylin­der in­sert.

LESS IS MORE

In fact, when I mea­sured the shot cy­cle, the ri­fle with the cylin­der in­sert re­coiled less, and had less surge than the ri­fle with the same pis­ton stroke and an ex­tended pis­ton rod or a pis­ton ex­ten­sion. The lower re­coil I can un­der­stand, be­cause the shorter pis­ton rod weighs less, but the lower surge is less eas­ily ex­plained; all I can think is that more of the com­pressed air is fol­low­ing the pel­let out of the muz­zle in­stead of driv­ing pis­ton bounce.

I am not com­mend­ing this mod­i­fi­ca­tion just yet, be­cause at the time of writ­ing. it has only been tested with a cou­ple of hun­dred pel­lets, and it re­ally needs in the re­gion of 10,000 shot cy­cles to show up any snags. All look­ing good so far, though.

VER­DICT

With most pro­duc­tion spring air­guns, a mod­est amount of lost vol­ume seems not in it­self a huge prob­lem, be­cause it rep­re­sents such a small per­cent­age of the swept vol­ume. With spring air­guns that have been mod­i­fied to re­duce swept vol­ume to a bare min­i­mum, as well as air­guns with low swept vol­umes to be­gin with, even mod­est lost vol­ume will un­doubt­edly be un­de­sir­able.

The ri­fle is very well be­haved with its polyurethane in­sert.

The ri­fle needs the same preload with an in­sert as it does with a longer rod or pis­ton ex­ten­sion.

It’s quite easy to cross thread the end cap, so fit it by hand, and care­fully.

Make sure that the end plug is screwed fully home; even a small gap will cause prob­lems.

When ma­chin­ing soft polyurethane, use sharp tools and low speeds.

The end plug is fit­ted with thread lock com­pound, which has to be soft­ened by ap­ply­ing heat here.

I used a mini-drill and stone to put a slight ta­per into the in­sert, which may or may not aid air flow.

Three meth­ods of short-stroking. The cylin­der in­sert is by far the cheap­est, and seems just as good.

To fit a pis­ton rod, you need a tool to keep it cen­tred whilst the thread lock com­pound cures.

I tried dif­fer­ent lengths and var­i­ous ax­ial hole sizes.

My pel­let start pres­sure ex­per­i­ments proved that mas­sive lost vol­ume was dis­as­trous to muz­zle en­ergy, and caused pis­ton slam.

With the scope back on, the ri­fle proved to be at least as ac­cu­rate with the cylin­der in­sert as with an ex­tended pis­ton rod.

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