AirGunTech: Re Engineering Spring Powered Air Rifles for smoothness, consistency, and to better suit UK power levels.

Helping out a mate with a seriously boingy HW80!

Shooting at the local club a few weeks back could not help but notice a mates HW80 was seriously boingy/twangy, in fact you could hear the spring twanging from 10 feet away!…so it was time to help.

The HW80 has a stock stroke of 80mm, hence its name, bore is 30mm for a swept volume of 56.6CC, this places it squarely in the magnum springer range designed for FAC use, so not really very good for sub 12FPE. I have tried everything possible to get the 80 shooting nice with a stock stroke, its always to lazy, short stiff springs, long weak springs..tried them all, so its time to short stroke.

With the gun being around 1 year old the compression cylinder is not honed true, this had me worried somewhat..however there was no need to worry. The internal cylinder was near on mirror finished and really quite true so the conversion to O ring seal was a go, stroke reduction to 55mm for 39CC swept volume (rifle is a .22)

The only other modification was to increase the transfer port from 3.2mm to 3.6mm, this allows the best from the 39CC and reduces felt surge, The spring is shortened to the length used for the HW35 with a new top hat machined from Acetal…power was 11.2FPE with Falcon 13.4Gr pellets, seriously easy to shoot with the only down side being the cocking arm only starts to push the piston back after the barrel has already travelled 30deg of its cocking cycle…not much of an issue for me but for some it may sway them against the modification.

Some pics of the piston just before installation.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

As you can see the piston is quite a bit longer than standard, 3 button bearings are added to the piston extension and a rear bearing added to the skirt of the piston…end result is no metal to metal contact, really very smooth cocking and firing cycle, dull thud, no hold sensitivity at all, fast lock time and very accurate. In reality this is much akin to transforming the HW80 into a HW35….still if it makes it easier to shoot and more pleasurable to shoot…why not! 😉

 

 

 

 

 

 

 

 

As mentioned a HW35 spring is what you need with this conversion, I would say the HW spring would be better than a bought kit spring as they usually are shorter and stiffer…and we still need some preload to counter piston bounce.

Now…My own HW80 is short stroked also although by 20mm not 25mm, my rifle having 42.4CC swept volume, i fitted a lazier spring to my gun and utilised an early HW90 barrel which has a set back cocking lever pivot point. Whether this feels better I will have to test when i get a chance to shoot my rifle against this gun here. Overall though a really nice end result with both conversions, safely below 12FPE and little swept volume to allow for power creep (especially on the 39CC piston) Ultimately both rifles feel like a full tuned hw95 to shoot…really quite special.

 

 

 

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