Tech­ni­cal Air­gun

Ba­sic Tech

Airgun World - - Contents -

Jim Tyler tests his newly short-stroked HW95 and cor­rects re­coil and hold­sen­si­tiv­ity prob­lems

To re­cap: I fit­ted the short­est pos­si­ble pis­ton ex­ten­sion to my HW95 to re­duce the avail­able pis­ton stroke to 77.5mm, in con­junc­tion with a light­ened pis­ton to re­duce re­coil, along with a soft spring that gave gen­tle re­coil. The aged and much-abused main­spring frac­tured, as de­scribed last month, but I had a re­place­ment, and was soon back in busi­ness. In the­ory, the re­duc­tion in stroke, pis­ton mass and soft spring should make the ri­fle as easy to shoot as any com­pa­ra­ble springer. The ri­fle felt good to shoot, and knocked down my prac­tice tar­get fine when shot off­hand so, with high hopes, I set about bench rest test­ing.

When test­ing ri­fles, I try to re­duce the ef­fect of my in­creas­ingly less than per­fect tech­nique (old age is a pain, but bet­ter than the al­ter­na­tive), by rest­ing my lead­ing hand on a pile of cush­ions that take the weight of the ri­fle, and also rest­ing my right el­bow on a cush­ion that af­fords some ‘give’ and al­lows my right arm to move for­ward and back with the re­coil and surge. Shoot­ing in the ex­act same man­ner, my TX200 matches my S510 in the ease with which I can place pel­lets pre­cisely, and I was ex­pect­ing much the same with the short stroked HW95.

OH DEAR

Af­ter roughly sight­ing the ri­fle and scope in, I shot a group at 40 yards with over 2” sep­a­rat­ing the most widely spaced pel­lets (which was most of them), so tried a dif­fer­ent type of pel­let, then an­other, and found that the group sizes were ap­palling with all of them.

I found that the ri­fle could put two, some­times three, pel­lets in close prox­im­ity, but then the point of im­pact (POI) shifted, seem­ingly in a ran­dom di­rec­tion, and it might then put two pel­lets on the new POI, be­fore shift­ing again. Shifts in POI be­tween se­quences of shots is by no means a rare phe­nom­e­non, and I be­lieve could be due to a range of is­sues.

Could the prob­lem be par­al­lax er­ror? The scope was a Nikon Prostaff 3-9 x 40 that I have fo­cused (for par­al­lax) at 25 yards, so the max­i­mum pos­si­ble par­al­lax shift at 40 yards is 6mm (ob­jec­tive lens radius mul­ti­plied by the dif­fer­ence be­tween the fo­cus dis­tance and tar­get dis­tance, di­vided by twice the fo­cus dis­tance), which in prac­tice is only achiev­able if you go out of your way to de­lib­er­ately look through the very edge of the scope, so the group size could not be ex­plained by par­al­lax er­ror. The Prostaff has high build qual­ity and, even though I did not sus­pect it to be faulty, I tried a sub­sti­tute scope to no ef­fect.

RE­TREAT!

De­spon­dent, I re­treated to the work­shop to check stock screw tight­ness, scope mount­ing

tight­ness, and si­lencer fit, all fre­quent causes of poor ac­cu­racy, but found no prob­lem with any of them. Then I had one of my in­fre­quent brain­waves, and re­turned to the range.

I rested the ri­fle’s fore end di­rectly on the pile of cush­ions, took the weight of the rear of the stock by cradling it us­ing the fin­gers of my left hand, barely touch­ing the pis­tol grip with my right hand, barely touch­ing the butt pad with my shoul­der, and shot a sat­is­fy­ingly tight group. There was noth­ing fun­da­men­tally wrong with the ri­fle; it was sim­ply too hold sen­si­tive for a shooter of my cur­rent abil­ity.

DÉJÀ VU

I’ve been here be­fore, when I re­duced the avail­able stroke of my .177 HW77 to 70mm, and found it to be the low­est re­coil spring air­gun I had shot to date, but amongst the least ac­cu­rate whereas, with the full 81mm of avail­able pis­ton stroke, it had been prob­a­bly the most ac­cu­rate. At the time, I re­stored the full 81mm of stroke, and ac­cu­racy was re­stored along with it.

Al­though I was able to mea­sure the re­coil cy­cle at the time I had the HW77 at 70mm stroke, the mea­sure­ments were crude com­pared to those I get to­day, thanks to hav­ing re­fined the equip­ment and work­ing meth­ods dur­ing the in­ter­ven­ing five years, and there is noth­ing in my data from the time that defini­tively ex­plained the hold sen­si­tiv­ity. My HW95 data, in con­trast, was re­cent, and far more ac­cu­rate, but there was still noth­ing un­to­ward in the data that I could see.

Clearly, it was time to put my re­coil mea­sur­ing rig (ac­celerom­e­ter, lin­ear gen­er­a­tor and dig­i­tal record­ing os­cil­lo­scope), to one side, and to start us­ing hold sen­si­tiv­ity as a mea­sure of the shot cy­cle in­stead.

AT A STROKE

I re­moved the alu­minium pis­ton ex­ten­sion and re­turned the avail­able pis­ton stroke to the orig­i­nal 85mm. This im­me­di­ately al­lowed the pis­ton to gain more en­ergy from the main­spring, and gave around half a foot pound at the muz­zle ex­tra, even with the re­duced spring preload that re­sulted from los­ing the pis­ton ex­ten­sion so, with­out both­er­ing to mea­sure the re­coil cy­cle, I took the ri­fle to my range and found the hold sen­si­tiv­ity to be greatly re­duced. With the fore

end rest­ing on my left hand, the groups were around a quar­ter of the size they had been with the short stroke, which was a huge im­prove­ment, though still a lit­tle shy of what I be­lieved the ri­fle to be ca­pa­ble, and have achieved us­ing it in the past. The im­por­tant point was that the POI jumps had stopped, and it seems that my fail­ure to match pre­vi­ous group sizes is al­most cer­tainly due to me, and not the ri­fle.

When the HW95 was my main ri­fle, I was spend­ing a huge amount of time shoot­ing it on the range to test mi­nor mod­i­fi­ca­tions, and the more time you spend shoot­ing a ri­fle, the bet­ter your marks­man­ship with it; it’s all about tech­nique and re­peata­bil­ity and, with­out that level of prac­tice, I was less able to repli­cate ex­actly the same hold shot af­ter shot. The in­creased hold sen­si­tiv­ity caused by the re­duced pis­ton stroke pun­ished even tiny flaws in my tech­nique.

CO­NUN­DRUM

By length­en­ing the avail­able pis­ton stroke, I had length­ened the com­pres­sion stroke and, with it, the pri­mary re­coil dis­place­ment, and it un­de­ni­ably re­duced the hold sen­si­tiv­ity. This flies in the face of the pop­u­lar be­lief that hold sen­si­tiv­ity is purely a func­tion of re­coil, and that more re­coil means greater hold sen­si­tiv­ity, which seems to un­der­pin the ma­jor­ity of the af­ter­mar­ket spring air­gun tun­ing busi­ness, which ded­i­cates its ef­forts to re­duc­ing re­coil in the be­lief that re­duc­ing re­coil will re­duce hold sen­si­tiv­ity.

This presents some­thing of a co­nun­drum, be­cause I’m ut­terly con­vinced that short stroking the HW95 (and, be­fore it, my HW77) in­creased hold sen­si­tiv­ity, whereas many tuners (and their cus­tomers) seem ut­terly con­vinced that short stroking re­duced hold sen­si­tiv­ity, so which is cor­rect? The an­swer could be ei­ther, nei­ther, or both.

When I’d shot the ri­fle off­hand, it had seemed fine, with no hint of hold sen­si­tiv­ity, but then my ex­pec­ta­tions weren’t ex­actly high; if I man­aged to reg­u­larly hit a 35mm kill zone at 30 yards, that was well up to scratch, for me, at least. Also, I’d asked an ac­com­plished HFT shooter to try the ri­fle, which he had done shoot­ing prone with a peg for his lead­ing hand and, al­though the ri­fle was shoot­ing left for him, the group­ing was

good. So, the self-same ri­fle could be deemed to be more or less hold sen­si­tive ac­cord­ing to who was shoot­ing it, and how.

DE­GREE

All re­coil­ing spring air­guns move while the pel­let is trav­el­ling up the bar­rel, and so all will re­spond to how they are sup­ported and re­strained dur­ing the re­coil cy­cle, so all must be hold sen­si­tive to some de­gree. My cur­rent ‘go to’ ri­fle is my TX200 Mk.3 with a mod­est stroke re­duc­tion (85mm) which, as I’ve al­ready stated, is as easy for me to shoot to a given stan­dard as my S510, so its hold sen­si­tiv­ity must be right at the bot­tom end of the scale, as was my 25mm cylin­der, 81mm stroke HW77 be­fore it; both ri­fles are hefty at around 4Kg, both have pis­tons that I class as light­weight (219g for the HW77, 220g in the TX), and both are fit­ted with soft main­springs which rely on preload to achieve the de­sired muz­zle en­ergy.

POS­SI­BLE CUL­PRIT

My TX200 and HW77 have one more thing in com­mon, and it is their gen­tle ini­tial pis­ton ac­cel­er­a­tion, which is a con­se­quence of hav­ing soft, pre­loaded main­springs, and which is the op­po­site of the faster pis­ton ac­cel­er­a­tion of my short stroked HW95 which, if not head­ing the list of sus­pects for hold sen­si­tiv­ity, is most prob­a­bly a con­trib­u­tory fac­tor, even if the cause is a sec­ondary ef­fect, and I do have a pos­si­ble cul­prit in mind.

Com­par­ing the re­coil traces of the HW95 fit­ted with a 77.5mm short stroke ex­ten­sion against those of my 90mm pis­ton stroke LGV, which ap­pears far less hold sen­si­tive, re­vealed a pe­riod of vi­bra­tion dur­ing the HW95’s pis­ton ac­cel­er­a­tion, and an­other pe­riod dur­ing pis­ton bounce, when the pel­let ex­its. One or other of these pe­ri­ods of vi­bra­tion might well con­trib­ute to hold sen­si­tiv­ity.

EX­TREME

The most ex­treme com­pres­sion stroke main­spring vi­bra­tion I have recorded was one of two ex­am­ples of the same ri­fle brought to me for test­ing. One was nor­mal stroke and an ab­so­lute pussy­cat to shoot, the other short stroked and a nasty, snappy al­ley cat in com­par­i­son. The re­coil traces for the two ri­fles are shown in the illustration ‘Pis­ton stoke and vi­bra­tion’. Sadly I was not able to test for hold sen­si­tiv­ity with the ri­fles, but if I can repli­cate the ex­treme vi­bra­tion by mod­i­fy­ing a ri­fle of known low hold sen­si­tiv­ity, it might prove or dis­prove whether the vi­bra­tion is the cause of, or a con­trib­u­tory fac­tor in, hold sen­si­tiv­ity.

So what caused the vi­bra­tion? In my HW95, short­en­ing the avail­able pis­ton stroke ne­ces­si­tated get­ting the pis­ton up to speed quicker, and it’s dif­fi­cult to con­clude other than the ini­tial pis­ton ac­cel­er­a­tion was the cul­prit for both the vi­bra­tion, and in­creased hold sen­si­tiv­ity.

CON­CLU­SION

In con­clu­sion, it seems the ac­cepted wis­dom that hold sen­si­tiv­ity is purely a func­tion of the length of the pis­ton stroke, and that longer strokes mean greater hold sen­si­tiv­ity, is plain wrong or, at best, a gross over-sim­pli­fi­ca­tion. Restor­ing my HW95 to its full orig­i­nal pis­ton stroke gave a marked de­crease in hold sen­si­tiv­ity, but whether pis­ton stroke is the full story re­mains to be seen.

As for me, what’s needed to en­sure full ef­fi­ciency is as ob­vi­ous as it is sim­ple – I need to prac­tise more.

Shoot­ing the ri­fle with my left hand sup­port­ing the butt re­duced hold sen­si­tiv­ity.

The Nikon Prostaff 3-9x40 is a lovely scope, but a heav­ier scope might re­duce hold sen­si­tiv­ity.

I like to set my springers up with soft springs and high preload, for a soft re­coil.

Restor­ing the full 85mm of stroke al­lowed me to re­move a coil from the spring, which helped the shot cy­cle.

My other HW95 has a full 85mm stroke, and is far less hold sen­si­tive.

The pis­ton ex­ten­sion is as short as pos­si­ble, but in­creases hold sen­si­tiv­ity.

My days of off­hand pest con­trol shoot­ing are now just a dis­tant mem­ory.

My mod­i­fied 90mm stroke LGV has hardly any vi­bra­tion dur­ing the com­pres­sion stroke, and none fol­low­ing pis­ton bounce. Very re­fined.

My 77.5mm stroked HW95 ex­hibits sig­nif­i­cant vi­bra­tion dur­ing the com­pres­sion stroke, and af­ter pis­ton bounce.

The same model of air­gun pro­duced pro­nounced main­spring vi­bra­tion when short stroked, but very low vi­bra­tion at full stroke.

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