Tech­ni­cal Air­gun

Ba­sic Tech

Airgun World - - Contents -

Jim Tyler ex­plains how ‘ex­treme tun­ing’ can po­ten­tially dam­age your ri­fle - and dis­pels some of the myths sur­round­ing it

I’m no great fan of the use of analo­gies to ex­plain any­thing to do with airguns, but there is an ex­cep­tion, and it is in one im­por­tant re­spect com­par­ing the spring air­gun to the in­ter­nal com­bus­tion en­gine. A pro­duc­tion spring air­gun is very like a pro­duc­tion car en­gine, in that the air­gun is de­signed to give many tens of thou­sands of shot cy­cles with only oc­ca­sional ser­vic­ing, and the pro­duc­tion en­gine should be good for 100,000 miles and up­wards, again with in­fre­quent ser­vic­ing. In con­trast, a highly tuned air­gun is like a highly tuned ‘race’ en­gine, which has to re­ceive ma­jor at­ten­tion at fre­quent in­ter­vals, up to and in­clud­ing a full re­build af­ter ev­ery sin­gle race; take things too far when tun­ing springers and they will be the same, but what is ‘tak­ing things too far’ in air­gun tun­ing? Well, it’s prac­ti­cally any­thing and ev­ery­thing that tuners do and have done in the past, from cram­ming in mon­ster main­springs in the early days of tun­ing, to the cur­rent fad of starv­ing the gun of pis­ton or air mass, but in both cases, taken to ex­tremes.


Prior to the early 1980s, a sur­pris­ing num­ber of airguns pro­duced in the re­gion of 8 to 10 ft. lbs., but be­cause very few of those who worked on airguns had the means to mea­sure muz­zle en­ergy, they did not know what muz­zle en­ergy they’d started with, nor what they ended with, so many blindly fit­ted a much stiffer spring, and if the re­sul­tant re­coil cy­cle was se­vere enough, judged the ‘power’ to have been in­creased. In fact, far from in­creas­ing muz­zle en­ergy, the con­se­quences of over-spring­ing were of­ten to lower the muz­zle en­ergy, whilst mak­ing the guns less ac­cu­rate, less re­li­able, prone to main­spring and other me­chan­i­cal fail­ure, and hope­lessly in­ac­cu­rate in many cases.

So, what was hap­pen­ing in­side the over-sprung airguns of old? The spring was ex­pand­ing ex­tremely rapidly, in­creas­ing the sever­ity of the re­coil, gen­er­at­ing higher cylin­der pres­sure faster than the air could es­cape through the trans­fer port. Af­ter the pis­ton and spring ran out of en­ergy, this re­sulted in se­vere pis­ton bounce and hence surge, fol­lowed by a very harsh pis­ton land­ing at the end of the sec­ond for­ward stroke. Many of the airguns of the time had a de­sign char­ac­ter­is­tic that limited their muz­zle en­ergy, and that was the limited avail­able length for the pis­ton stroke which, cou­pled with the high start pres­sure pel­lets of the time, gave a very short and brief cylin­der pulse – the length of pis­ton travel and time that the pis­ton and pel­let are trav­el­ling in the same di­rec­tion, when the pel­let needs to gain most of its ve­loc­ity.


The only sure method of rais­ing muz­zle en­ergy in many airguns at the time, with­out ma­jor surgery, i.e. length­en­ing the pis­ton stroke where pos­si­ble, was to en­sure that au­toigni­tion (dieseling) gave a pres­sure and tem­per­a­ture spike to get the pel­let mov­ing ear­lier, to in­crease the cylin­der pulse and give the pel­let time to ac­cel­er­ate to a de­cent ve­loc­ity be­fore the pis­ton bounced.

Be­cause most airguns were fit­ted with leather pis­ton seals that could act as a wick for the oil that fu­elled dieseling, it was com­mon prac­tice to use a light min­eral oil, with a low au­toigni­tion tem­per­a­ture, for cylin­der

“… This is not to be seen as a ‘cure’ so much as a stick­ing plas­ter for a self-in­flicted wound …”

lu­bri­ca­tion. Be­cause the au­toigni­tion nec­es­sary to get the muz­zle en­ergy of short stroke airguns to the de­sired level oc­curred at rel­a­tively low tem­per­a­ture and pres­sure, over-spring­ing the air ri­fle was to­tally un­nec­es­sary. It served only to make the ri­fle more dif­fi­cult to shoot ac­cu­rately, as well as raise peak cylin­der tem­per­a­ture and in­crease the chance of spring fa­tigue or fail­ure, but be­cause it in­creased re­coil and surge, peo­ple con­tin­ued to cram in far too much spring in the mis­taken be­lief that the fe­ro­cious re­coil was a sure in­di­ca­tor of ‘power’.

Af­ford­able, re­li­able and ac­cu­rate chrono­scopes be­came avail­able in the early 1980s, re­veal­ing that airguns fit­ted with overly long and stiff springs usu­ally did not achieve the de­sired muz­zle en­ergy, and they fell out of favour with tuners, though most af­ter­mar­ket springs were still stiffer and longer than was good for the guns in which they were fit­ted, a sit­u­a­tion that sadly con­tin­ues to this day.


Most peo­ple know that re­coil is a re­ac­tion to the mass of the pis­ton and spring pow­er­ing for­ward dur­ing the com­pres­sion stroke, and that re­duc­ing the pis­ton’s weight also re­duces re­coil dis­place­ment. If the weight re­duc­tion is mod­est, or if you’re pre­pared to sac­ri­fice a lit­tle muz­zle en­ergy, then no harm done, but go over the top at your peril.

Re­duc­ing the pis­ton mass re­duces the pis­ton’s mo­men­tum, which is a big deal be­cause it is the mo­men­tum that com­presses the air. So, if you want to main­tain muz­zle en­ergy, you need to in­crease the pis­ton’s ve­loc­ity by fit­ting a stiffer spring to get the mo­men­tum back up. Un­for­tu­nately, the lighter pis­ton has lower in­er­tia – re­sis­tance to mov­ing – and at the end of the com­pres­sion stroke, the high pres­sure air in the cylin­der ac­cel­er­ates it back up the cylin­der faster and fur­ther, low­er­ing muz­zle en­ergy and in­creas­ing surge. You can com­pen­sate for this by ei­ther in­creas­ing spring preload to re­sist the force of

com­pressed air, for which you’ll prob­a­bly need a softer, longer spring, or us­ing a stiffer spring to drive the pis­ton nearer the cylin­der end, in­creas­ing peak cylin­der pres­sure, and tem­per­a­ture. As the fash­ion to­day is for less preload, most go with the stiffer spring op­tion.

The is­sue with high peak cylin­der pres­sure and tem­per­a­ture will be­come ap­par­ent as you read through the ar­ti­cle.


Fric­tion, mainly be­tween the pis­ton seal and cylin­der wall, was for many years seen as the great­est ob­sta­cle to in­creas­ing air­gun muz­zle en­ergy, which has led many to ex­per­i­ment with a suc­ces­sion of su­per lu­bri­cants, some­times with un­ex­pected and un­de­sir­able con­se­quences.

The most com­mon ‘su­per’ lu­bri­cants are poly­te­traflu­o­roethy­lene (PTFE) and molyb­de­num disul­phide, usu­ally re­ferred to as ‘moly’; in ei­ther case, in the form of tiny par­ti­cles in a vis­cous liq­uid car­rier, and it is the car­rier that causes the prob­lems, rather than the lu­bri­cant. Some car­ri­ers give off fumes that ig­nite dur­ing the com­pres­sion stroke, just like airguns of old dieseling with light min­eral oil, and there’s a po­ten­tial prob­lem if the amount of fumes given off varies with tem­per­a­ture. If the fumes from one shot reached the sat­u­ra­tion level nec­es­sary for au­toigni­tion, but failed to achieve that with the fol­low­ing shot, there will be two very dif­fer­ent pel­let start­ing points, and two widely spaced pel­let points of im­pact.


The big­gest re­duc­tion in fric­tion comes from re­plac­ing a syn­thetic ‘para­chute’ pis­ton seal with an ‘O’ ring. Dur­ing the com­pres­sion stroke, as the air pres­sure rises, the syn­thetic pis­ton seal is squashed, and presses ever harder against the cylin­der wall, caus­ing a huge in­crease in fric­tion in the hun­dreds of New­tons – the fric­tion rises ex­po­nen­tially, along with cylin­der pres­sure. This does not hap­pen with an ‘O’ ring, so the com­pres­sion stroke is much faster, but – and it’s a big but – the fric­tion be­tween the syn­thetic seal and cylin­der wall that slows pis­ton bounce is not there, so the pis­ton bounces faster and fur­ther, start­ing when the pel­let would still be ac­cel­er­at­ing with a para­chute seal. To com­pen­sate, peak cylin­der pres­sure must be raised, which means peak cylin­der tem­per­a­ture is higher.


The eas­i­est way to re­duce air mass is to shorten the pis­ton stroke, which is cur­rently very much in vogue, and which re­duces re­coil, as well as quick­en­ing the shot cy­cle to pel­let exit. Be­cause most full-sized spring airguns are des­tined not only for the UK mar­ket, but also mar­kets where muz­zle en­ergy is un­re­stricted, most are de­signed with longer pis­ton strokes than strictly nec­es­sary for our 12 ft.lbs., and a mod­est re­duc­tion in stroke can be un­der­taken with­out any un­de­sir­able side ef­fects. How­ever, some tuners like to take things to ex­tremes, and go OTT.

Re­duc­ing pis­ton stroke to ex­cess is ef­fec­tively turn­ing the clock back to the bad old days of the late 1970s, when most air ri­fles had short strokes, and we all strug­gled to get them to pro­duce muz­zle en­ergy as al­ready de­scribed.


There is an­other way to re­duce air mass, which does not in­volve short­en­ing the stroke, and it is to fit a sleeve in the cylin­der to re­duce the di­am­e­ter. The at­trac­tion of cylin­der­sleev­ing is that it al­lows a less heavy pis­ton to be used, re­duc­ing re­coil, with­out the draw­backs al­ready de­tailed, and the rea­son why is that it main­tains the pis­ton’s sec­tional den­sity and re­sis­tance to bounce, whilst low­er­ing the force from the com­pressed air that causes bounce, which is pro­por­tional to the pis­ton’s cross-sec­tional area..

If you sleeved a 25mm cylin­der down to 23mm the force driv­ing bounce would re­duce ac­cord­ing to the square of the di­am­e­ter, and the pis­ton mass could be re­duced by the same amount whilst main­tain­ing sec­tional den­sity, so a 250g pis­ton could be re­duced to 211g, mak­ing a big re­duc­tion in re­coil. Sounds good, and doubt­less is up to a point, but if you go over the top, there’s the po­ten­tial for nasty things to hap­pen.

I have been fol­low­ing In­ter­net fo­rum posts from peo­ple us­ing sleeved cylin­der and short stroke airguns, and while many re­port no prob­lems, some de­scribe mul­ti­ple main­spring break­ages, oth­ers fried breech seals.


The re­ported breech seal dam­age is caused pri­mar­ily by ex­tremely high tem­per­a­ture, prob­a­bly ex­ac­er­bated by ac­com­pa­ny­ing ex­tremely high air pres­sure, and both are a re­sult of the tuner go­ing over the top to com­pen­sate for prob­lems of their own mak­ing, which can in­clude over-short­en­ing the avail­able pis­ton stroke, over­do­ing a re­duc­tion in pis­ton mass, or go­ing too far when re­duc­ing the swept vol­ume. There seems to be a re­la­tion­ship be­tween avail­able air mass and the max­i­mum muz­zle en­ergy that can be achieved be­fore peak tem­per­a­ture be­comes high enough to dam­age seals. I have wit­nessed breech seal fail­ure fol­low­ing the very first shot of an air­gun that was pushed be­yond the ‘nat­u­ral’ muz­zle en­ergy level for its swept vol­ume, and I have seen cu­mu­la­tive breech seal fail­ure with an air­gun that had a com­bi­na­tion of too light a pis­ton and too lit­tle air mass. In both cases, man­i­fest as a sud­den and large fall in muz­zle en­ergy, and in both cases, the seals looked as though some­one had played a blow torch on them.


Tem­per­a­ture re­lated breech seal fail­ure can be re­duced in some cases, or per­haps de­layed, by cham­fer­ing the trans­fer port out­let, which pro­vides a small vol­ume into which the ex­tremely high tem­per­a­ture air can ex­pand be­fore it reaches the seal and, on ex­pand­ing, the air cools. This is not to be seen as a ‘cure’ so much as a stick­ing plas­ter for a self-in­flicted wound; the only cure is to re­verse the causative mod­i­fi­ca­tion to an ex­tent. Give a lit­tle mass back to the pis­ton, give the gun more air mass by in­creas­ing the pis­ton stroke, and take away some of the spring en­ergy.


By no means all, but the ma­jor­ity of main­spring frac­tures seem to oc­cur about an inch from the rear of the spring, and one plau­si­ble ex­pla­na­tion is that the spring in this

“… the seals looked as though some­one had played a blow-torch on them …”

re­gion is still ex­pand­ing when the coils in front of it are com­press­ing af­ter pis­ton bounce, and the frac­ture oc­curs where the two wave fronts meet, prob­a­bly not on shot one, but fol­low­ing re­peated wave front col­li­sions. Although the wave front col­li­sions are a func­tion of tim­ing, there can be lit­tle doubt that the chances of frac­ture are in­creased the higher the stresses on the spring.

If the rea­son for the sleeved cylin­der ri­fles break­ing main­springs is in­deed col­lid­ing wave dis­place­ment fronts, then the cul­prit will al­most cer­tainly be fe­ro­cious pis­ton bounce due to very high cylin­der pres­sure at, and fol­low­ing, pis­ton bounce. Again, the so­lu­tion is to add back a lit­tle swept vol­ume, add back a lit­tle pis­ton stroke or pis­ton mass, and take some of the strain off the main­spring.


There is an­other po­ten­tial cause of high spring fail­ure rate, and it is coil di­am­e­ter. If the di­am­e­ter of a pis­ton is re­duced to, say 22mm, then the spring di­am­e­ter must also re­duce to fit in­side the nar­rower pis­ton, to the point at which the spring is sig­nif­i­cantly ex­ceed­ing its elas­tic limit in use, lead­ing to creep (short­en­ing of the spring with­out weak­en­ing it), fa­tigue (short­en­ing and weak­en­ing the spring), or break­age.

Per­haps to ad­dress the prob­lems with small di­am­e­ter springs, per­haps for ease of man­u­fac­ture, some tuners have ex­per­i­mented with ‘skirt­less’ pis­tons, which can only work in airguns that use a slid­ing cylin­der to com­press the main­spring. Skirt­less pis­tons are noth­ing new, but in the past, they have tended to fea­ture in very cheaply made, low muz­zle en­ergy airguns, with springs that were not over-taxed, and my con­cern with skirt­less pis­tons in airguns pro­duc­ing near 12 ft.lbs. is that if the front end of the spring is poorly sup­ported, it might go out of line and adopt the fa­mil­iar ‘S’ shape more quickly. Time will tell whether my con­cern is founded.

Per­haps when it comes to spring air­gun mod­i­fi­ca­tion, the old adage, ‘Ev­ery­thing in mod­er­a­tion’ might be worth bear­ing in mind.I

The Orig­i­nal (Diana) 45 was the first true mag­num air­gun de­signed for un­re­stricted mar­kets, and the first I short-stroked for UK lev­els.

Prior to the 1980s, the Air­sporter main­spring was widely believed to raise the muz­zle en­ergy of any spring air­gun. It did not.

Go too high with peak cylin­der pres­sure and tem­per­a­ture, and you can scorch breech seals. Spring break­age is com­monly caused by in­ap­pro­pri­ate heat­ing and cool­ing dur­ing short­en­ing, although fe­ro­cious pis­ton bounce can also be re­spon­si­ble.

Ex­treme cylin­der tem­per­a­ture and dieseling do pis­ton seals no good at all.

Tak­ing in­ter­nal changes too far can make ri­fles much harder to shoot well.

Man­u­fac­tur­ers as well as tuners oc­ca­sion­ally go OTT, such as the 22” bar­rel of the HW35 Ex­port.

If there’s in­suf­fi­cient swept vol­ume, no spring is go­ing to achieve a de­sired muz­zle en­ergy.

The fash­ion is for stiff springs with min­i­mal preload, which cer­tainly makes as­sem­bling airguns eas­ier, and safer.

Square sec­tion springs used to be com­mon­place, usu­ally crudely ground down to be crammed into an un­for­tu­nate air­gun. Bad idea on ev­ery level.

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