Tech­ni­cal Air­gun

Ba­sic Tech

Airgun World - - Contents -

Jim Tyler helps to soft-tune a Walther LGV to achieve an ‘out of this world’ stan­dard of ac­cu­racy

Ire­mem­ber read­ing the edi­tor’s review of the Walther LGV which was, as far as I’m aware, the first pub­lished review, and be­ing so sur­prised at the lav­ish praise Terry heaped upon the ri­fle that I called him and asked whether it re­ally was that good. There was no dis­guis­ing the en­thu­si­asm in his voice, and I knew that the ri­fle re­ally was as good as he’d said.

There were a few naysay­ers on the In­ter­net, of course, peo­ple who had never shot the ri­fle and yet knew it wasn’t as good as Terry had in­di­cated, but Terry took the test ri­fle to a num­ber of shows, and the some­times vo­cif­er­ous naysay­ers who tested it on the range were strangely sub­dued after­wards. Then it was my turn.

I tested the ri­fle’s ac­cu­racy; stripped it down, weighed and mea­sured ev­ery­thing, and pub­lished my find­ings, in­clud­ing the di­men­sions of the trans­fer port which, at 26.9mm x 2.6mm, seemed to run against com­monly accepted wis­dom that trans­fer ports should be as short as pos­si­ble. I freely hold my hands up to dis­cov­er­ing a re­la­tion­ship be­tween port length and en­ergy ef­fi­ciency the year be­fore when test­ing my HW77, and pub­lish­ing it, and it seems a lot of peo­ple had taken that as gospel, judg­ing by the re­ac­tion on the In­ter­net to the LGV’s trans­fer port di­men­sions.

It wasn’t just the trans­fer port’s length that peo­ple seized upon; it was also the 2.6mm di­am­e­ter, which was widely thought to re­strict the flow of air, re­duc­ing the muz­zle en­ergy, and some went as far as to spec­u­late that this placed huge strain on the main­spring, dam­ag­ing it. The truth is that we don’t know enough about the physics of the spring air­gun (most es­pe­cially the lit­tle re­searched field of com­press­ible fluid dy­nam­ics) to reach such con­clu­sions, which I be­lieve are both ques­tion­able.

Pos­si­bly due to peo­ple think­ing the trans­fer port length was pre­vent­ing the ri­fle reach­ing its full po­ten­tial, the tun­ing com­mu­nity don’t seem to have spent much time work­ing with the LGV, pos­si­bly con­cen­trat­ing of the un­der lever LGU, HW97 and TX200 in­stead. That’s a shame, be­cause the LGV is a ri­fle with great po­ten­tial, de­spite the long trans­fer port.

CROSS PUR­POSES

Af­ter pub­lish­ing my test find­ings on the LGV, I went back to work­ing with my then two main ex­per­i­men­tal ri­fles, the HW77 and HW95, and the LGV took very much a back seat for some years, un­til very re­cently. A mem­ber of the ex­cel­lent Shoot­ing the Breeze (STB) web site (https://shoot­ing-the-breeze.com/) con­tacted me with a ques­tion about al­ter­ing the trans­fer port di­am­e­ter of the LGV, but – me be­ing me

– I read it as a ques­tion about the LGU, which gen­er­ates far greater in­ter­est from the tun­ing per­spec­tive, and an­swered the ques­tion, adding my thoughts on a suit­able al­ter­na­tive main­spring, based on re­cent ex­per­i­ments with my LGU. It was not un­til the mem­ber came back to me that I read the mes­sage prop­erly, and re­alised he had an LGV, had fit­ted the spring I’d rec­om­mended, and re­ported that the re­sult was ‘out of this world’. I did the maths, and the spring I had rec­om­mended had made 29 ft.lbs. avail­able to the pis­ton, which gives a muz­zle en­ergy of 11.6 ft.lbs., for an ef­fi­ciency of 39%, which is good for a break-bar­rel, and rather sug­gests that the long and thin trans­fer port is not sig­nif­i­cantly sti­fling the muz­zle en­ergy. The spring in ques­tion is prob­a­bly the best off-the-shelf main­spring in terms of qual­ity and longevity that I’m aware of, and it’s the fac­tory stan­dard Mk.3 Air Arms TX200 spring that works well in my LGU, and which is a very dif­fer­ent beast from the stan­dard Walther spring. Both springs are 20.8mm ex­ter­nal di­am­e­ter, but that’s just about all they have in com­mon; the TX spring has just 23 coils against the Walther’s 32, the TX wire is 2.9mm against the Walther’s 3.25mm, and the TX spring rate is 5.36 N/mm against the Walther’s 6.39 N/mm.

The stan­dard Walther spring when new has around 26mm of preload, and the trick when

“in this case it was the heav­ier pel­let that pro­duced the bet­ter ac­cu­racy”

switch­ing to a softer spring is to in­crease preload, which will have a fun­da­men­tal ef­fect on the shot cy­cle, and even how the ri­fle man­ages to achieve its muz­zle en­ergy.

The stiffer spring makes more en­ergy avail­able to the pis­ton, so the pis­ton stroke in­creases slightly, in­creas­ing the peak cylin­der pres­sure and tem­per­a­ture, but once the pis­ton comes to a halt, the greater pres­sure from the com­pressed air re­main­ing in the cylin­der is met with less op­pos­ing force from the less pre­loaded main­spring, so the pis­ton bounces faster and fur­ther. The softer spring makes less en­ergy avail­able to the pis­ton, so the pis­ton travels a frac­tion less dis­tance so that when it comes to a halt, there is lower air pres­sure to drive bounce against a higher op­pos­ing force from the greater preload, so the pis­ton bounces more slowly and less. The shot cy­cles have a fun­da­men­tal ef­fect of felt re­coil.

Most peo­ple shoot­ing a spring air­gun fit­ted with a stiff spring re­mark that it feels ‘quick’, but whether they are sens­ing a dif­fer­ence be­tween, say, nine and a half and 10 mil­lisec­onds, or whether they are sens­ing frac­tion­ally greater re­coil travel or, and most likely, they are sens­ing faster re­coil ac­cel­er­a­tion, is open to ques­tion. I very much sus­pect it’s the lat­ter. In con­trast, a soft, heav­ily pre­loaded spring gives a gen­tle re­coil ac­cel­er­a­tion, so some will ob­serve that it feels ‘slow’.

SIZE IS­SUE

The TX200 spring might be the same out­side di­am­e­ter as the Walther spring, but it has thin­ner wire, so its in­ter­nal di­am­e­ter is greater, which makes it a slack fit on the LGV’s spring guide. The spring is thus fairly free to vi­brate, which is eas­ily damp­ened with grease, but there’s an­other is­sue; the spring can buckle just enough dur­ing the lat­ter stage of the cock­ing stroke to con­tact the pis­ton wall which, not be­ing very smooth, makes the cock­ing stroke feel slightly rough. The so­lu­tion is to make, or have made, a cor­rectly sized spring guide and top hat, which I will get around to in the full­ness of time.

PER­FOR­MANCE

With 40mm of preload, the ri­fle was far too close to the le­gal limit with Air Arms Field and Express, so I re­duced the preload by de­grees, stop­ping at 36mm, which gave 11.3 ft.lbs. at the muz­zle, which with Air Arms Field meant a muz­zle ve­loc­ity a few fps ei­ther side of 778fps, pretty much per­fect for HFT, for which the LGV is prob­a­bly the best suited break-bar­rel, fully ca­pa­ble of mix­ing it with the heavy un­der-levers that dom­i­nate the HFT Re­coil class.

My test rig mea­sured re­coil at just un­der 5mm, which would re­duce to less than 4.5mm with a 0.5kg scope, with pel­let exit (AA Field) oc­cur­ring just 0.2mm into the to­tal of 1.04mm of surge. The time of the en­tire shot cy­cle to pel­let exit was 11.22 mil­lisec­onds (ms), the com­pres­sion stroke took 9.9ms with an av­er­age pis­ton ve­loc­ity of 8.9 me­tres per sec­ond (M/s), peak pis­ton ve­loc­ity of 14.95 M/s, and pis­ton land­ing of just un­der 4 M/s. The pis­ton land­ing ve­loc­ity is rather higher than I ex­pected, pos­si­bly a side ef­fect of the trans­fer port di­am­e­ter caus­ing mass air flow to choke a lit­tle too early in the stroke. With 7.8g Air Arms Express pel­lets, the pis­ton land­ing ve­loc­ity was more in line with ex­pec­ta­tions at 3.1 M/s.

Af­ter the pis­ton lands, any fur­ther ri­fle move­ment/vi­bra­tion can only be caused by the main­spring throw­ing its weight around, and in this re­spect the shot cy­cle was rather more re­fined with 7.8g pel­lets. With the 8.4g (AA Field) pel­lets, mi­nor vi­bra­tion con­tin­ued up to the point at which the pel­let will have trav­elled ap­prox­i­mately 25 yards, but with the 7.8g (AA Express) pel­lets, all move­ment and vi­bra­tion ceased when the pel­let had trav­elled just five yards which, in the­ory, has to be good for ac­cu­racy. The­ory and prac­tice don’t al­ways agree, though, and in this case it was the heav­ier pel­let that pro­duced the bet­ter ac­cu­racy.

TRANS­FER PORT

Mike Wright con­cluded that al­ter­ing the di­am­e­ter of the trans­fer port should be the last stage in tun­ing a spring air­gun, and that’s my

“it boils down to us­ing gut feel­ing to de­cide which seems most promis­ing”

ap­proach. The main ef­fect of al­ter­ing the trans­fer port is not, as seems log­i­cal, to al­low more air through the big­ger hole to pro­pel the pel­let, but is to al­ter the point in the shot cy­cle at which the mass air flow chokes and with this par­tic­u­lar LGV tune, I want to de­lay chok­ing to bet­ter suit the 8.4g pel­lets, which will mean en­larg­ing the port.

What size to make the port, though? I do have three for­mu­las based on match­ing the cylin­der and bar­rel pulses, match­ing the pis­ton and pel­let ve­loc­ity, or tak­ing the pulse as a per­cent­age of the stroke – all at the point of pis­ton bounce, but al­though some of my recorded mea­sure­ments are used, all three for­mu­lae are based to a de­gree on a num­ber of ed­u­cated guesses, such as how far the pel­let is up the bar­rel at pis­ton bounce, so this is not an ex­act sci­ence. The three for­mu­las al­ways pro­duce three dif­fer­ent re­sults, any­way, and in the end it boils down to us­ing gut feel­ing to de­cide which seems most promis­ing.

Match­ing the ve­loc­i­ties of the pis­ton and pel­let at pis­ton bounce gave a sug­gested port di­am­e­ter of 2.9mm; match­ing the pulses sug­gested 3.26mm, and tak­ing the cylin­der pulse as a per­cent­age of the stroke sug­gested 3.1mm. The for­mu­las take no ac­count of trans­fer port length and with the LGV trans­fer port be­ing very long and hence hav­ing a large vol­ume, I’ll prob­a­bly opt for a con­ser­va­tive 2.9mm. That’s for the fu­ture, though, for two rea­sons; first, I don’t have a drill bit suit­able at the present, and sec­ond, the ri­fle needs more thor­ough test­ing as it is be­fore al­ter­ing any­thing else, which will en­tail a lot of range work, and some HFT com­pe­ti­tion.

If you want to try the shot cy­cle from a softer spring in your LGV, the Mk.3 TX200 spring fits the bill nicely, al­though you will need preload wash­ers and, to get the best out of it, new spring guides.I

A soft spring and a long bar­rel give low cock­ing ef­fort.

My re­coil mea­sur­ing set-up sug­gests this LGV tune holds much prom­ise.

My low-tech so­lu­tion to op­er­at­ing the trig­ger with­out mov­ing the ri­fle when re­coil test­ing.

The man­ual bar­rel lock is rock solid.

With 36mm of preload, it takes lit­tle force (42 lbf) to fit the spring.

The shot cy­cle is gen­tle with Air Arms Field, and the ac­cu­racy ap­pears sec­ond to none amongst sport­ing spring air­guns.

With Air Arms Express, surge is less than that with the heav­ier Field.

The dig­i­tal vernier caliper is in­valu­able for mea­sur­ing things like preload.

Looks like a piece of grit found its way un­der the cock­ing arm bear­ing at some point. That needs pol­ish­ing out.

The cap­tive spring guide is about half a mil­lime­tre less than ideal in di­am­e­ter for the TX200 spring.

The LGV pis­ton seal works well enough; some peo­ple fit af­ter­mar­ket seals, which can im­prove en­ergy ef­fi­ciency a tad.

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