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

HW95, Tuning for dedicated UK spec.

Lucky for me there are a few guys willing to send in their gun so I can tinker a little…obviously I have to return them shooting better than they did when they arrive ;). This time round (in between creating another HW80/LGV Hybrid) I’m going to take a look at the HW95.

The example sent to me is a slightly older one, not the newer spec with new style cocking shoe etc, the gun is like brand new however with very little wear signs…so no repairs to do 😉

Measurements taken put the piston stroke at 85mm, 26mm piston, 45.1471428CC swept volume…

The transfer port is 3mm diameter and 15mm long (to long really HW…stick to 10mm or less if you can ;)) 0.106071CC

Out the box we have a Static compression of 425:1. NOT counting the lost volume in the parachute grooves on the piston seal.

The plan is to bring this HW95 to pure UK spec, max efficiency at around 11.3FPE, so my plan is to short stroke the piston down to 74mm, I will do this with an Alu piston cap and O ring seal to minimise lost volume, adding bearings to this cap (should have enough room) or may even go with a full circumference bearing. The piston will be sleeved and a rear bearing added so ensuring zero metal to metal contact. The transfer port will be opened to 3.3mm and polished, couple this to a reduction is swept volume to around 39CC we should see 306:1 static compression ratio. The flow through the transfer port will be enhanced by the larger diameter so I have no fear the action will still make good UK spec power levels…the plan is for it to land pretty much bang on 11FPE or so with a 230mm spring 3mm wire 31 active coils (33 counting the flattened ends) with little to no added preload over the stock amount…we shall see 😉

This may be a slow project as a few other jobs have to be finished up to free up time…Once the time is available you should see a full update.






  • Forgive me for not being fully up to speed here but, if you simply just opened up the TP to say 4.1mm diameter that would give you the sort of compression ratio you want. The swept volume would remain the same so impact upon lock time would be marginal. This lock time might be then improved by fitting an O seal of the sort that Vortek now do for the TX (I know they don’t do one yet)?

    Also, not knowing the underlying formulae, what FPE is the 425;1 and 45cc swept volume actually mathematically optimal for ?

    enjoy following your exploits – thanks.

  • Brian…the static compression ratio has nothing to do with final FPE produced, it is related to the distance from the breech face the piston will travel to before it bounces.
    45CC is for FAC velocities (mainly the USA market), for sub 12FPE 39CC or under is more suitable.

  • OK so optimum comp ratio should be roughly the same whatever the power. Swept volume is increased cos you just have to blow for longer to get the pellet to go faster ? For us here in Blighty blowing too hard/long is inefficient and gives us an unnecessarily long lock time.

    So if I was in the USA (or for FAC use in UK) I would simply be looking to ream out my TP to get the rifle into a sweeter CP band. Is there a proven relationship between TP diameter and calibre, I am thinking .20 here.

  • Brian, what you need to consider is how much swept volume you need to push the pellet at 11fpe, then you need to consider piston weight and where that piston is going to bounce on the compression stroke…this also dictates the flow rate thru the transfer port.

    11fpe in .20 will need around 38CC of air, reverse engineer that to give you the stroke you need, now optimise the transfer port to give the static compression that will allow the piston to fly almost to the end of the compression stroke and still have enough pressure to cushion the piston to a stop that feels sweet.usually this is around 300:1, you can go lower BUT to low you induce piston slam. 300:1 is safe.

    So you now need to think HW95 is 26mm bore, 39CC is 74mm stroke, so 38 is going to be around 73mm or so. My calculation gave 306:1 with 3.3mm, with less swept volume the SCR will decrease a little so you may be looking at 3.2mm being optimum. Being honest if you ran 39CC like i calculated above the rifle would still be optimally set up for UK use in .20 just like .177.

    I start the 95 this week, so if you are patient I will be updating with some results after i short stroke the piston down and button it…you could then see if its worth copying what I have done.

  • I can’t seem to find the lgv piston diameter. Would it possible to fit these pistons into hw95s?

    Great blog btw.

  • The LGV piston will not fit in the 95 as it has a shouldered skirt.

    The dia of the LGV piston is 24.8mm, the shoulder is 29.8mm

  • Any progress on this? I’ve got an HW95k in .177 with the old style cocking shoe and 310mm screw cut barrel with silencer. She kicks like a mule though. Balancing a pellet on top of the scope and firing results in it ending up in the neighbours garden! Occasionally I can get good groups but it’s incredibly hold sensitive and difficult to get any sort of consistency out of… I’d be interested in what sort of results you got from this tune. Was it .177 or .22 by the way?

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