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.
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.
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.
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.
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.
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.
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.
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.
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.