Short stroking a Webley VMX .22

[Wargruf]

Hi there guys an gals,
 I’m looking into short stroking my .22 Webley Vmx, farm yard weapon of choice.

Now I think I’ve got my maths correct on this, but if anyone here’s got some pointers or a bit more technical experience the information would be great...

Right then the stock setup is as follows,
Transfer port is 4.2mm diameter 13.5mm long: 0.1802577cc
Cylinder has 100mm travel: 57.26cc

57.26/0.1802577= 317.65:1 static compression ratio (scr)

This works ok, but because I’m based in the UK and the ft/lbs are restricted to 12ft/lbs the shot cycle is slower and has a good kick to it. The rifle is quite hold sensitive in this setup but it is accurate using the artillery hold.

What I’m planning on doing is fitting a longer softer spring from Titan Xs, their new Hatsan one that’s come out.
OD19.55mm, ID13.4mm, Wire 3mm, 37 Coils and 310mm long.
(The spring in at the moment is a good 55mm shorter and has less coils so is stiffer.)

That and hopefully Tinbum tuning, from the uk, can make me fitted guide and top hat along with a whopping 40mm spacer for the front of the piston.
I’ve already lightened the piston a little bit and I’m looking toward 250-225g when I finally finish putting holes in it and I’ve sleeved the transfer port to 3.3mm still 13.5mm long : 0.11470945cc
The new short stroke piston extension should reduce my swept volume to 34.35cc or 60mm travel.

34.35/0.11470945= 299.45:1 SCR

This should speed up the locktime and reduce the recoil by a good margin I hope..?

Anyhow, hopefully I’ll get a price back from Tinbum tuning and get the parts ordered for a reasonable sum if you guys think this new setup sounds like it’s on the money.

Cheers for reading,
Gruf

[DRose]

Good luck with your tune. Sounds interesting. Dave

[DanD]

I'm definitely interested in your results.
How did you sleeve the transfer port?

[subscriber]

Interesting Craig,

Your plan sounds simple enough, and is totally logical. 

Performance expectations:
Your modification should certainly reduce lock-time and change the feel on firing.  Will it match the performance you have now?  You are going to find out. 

It may be smart not to take too much weight out of the piston initially, as too light a piston rebounds more against the high pressure air column behind the pellet.  This can reduce performance, especially with heavier pellets.  The higher preload on the spring should offset the lighter weight of the piston to some degree, but perhaps not as much as one might suppose.

I think you might find this study on the effects of spring air rifle, piston and port dimensions interesting reading:  https://www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns


One aspect that "springs to mind" is that there will be a lot more pre-load on your replacement spring, using a "long" piston.  This should not be a problem with a proper spring compressor. 

I assume you have calculated that the replacement spring will allow the piston to reach the cocked position (plus a little travel to engage the sear) without going into coil bind (coils touching)?

If the original rifle was designed to be assembled only with a spring compressor, then that aspect should be unchanged.  Contrast this with something like the Slavia 634:  A 10 ft.lb air rifle that has so little preload on the spring, you can close the endcap by hand.  As the Slavia's endcap screws on, using the hand is the standard method. 

Else, a lathe tailstock with live center could be used.  That would allow rotation (if required), while also enabling the spring to be compressed in a controlled manner.  I mention this detail, only in case you have not figured out your spring compressor yet...

[subscriber]

Hi again, Craig

If I calculated correctly, you piston diameter is 27 mm.  That has .8878 square inches of area. 

So, if you are relying on "high spring force" near the end of the stroke to produce pellet driving pressure, 100 lb (45.45 kg) of spring preload would produce 113 PSI (7.687 BAR).  That would produce a 4.3 lb force when acting on a .22 pellet area. 

If pellet to barrel friction is 2 lb, then the net force accelerating the pellet from residual spring load is only 2.3 lb.  Or if your spring preload at the end of the stroke is double that, then double these numbers.

Based on the above, it should be easy to see that the bulk of the pellet's energy comes from the piston's kinetic energy:  Piston velocity gained in the first 80 % of the stroke, where the steel spring had high compression and the "air spring" slowing the piston had very low compression.  This kinetic energy of the piston is suddenly converted to air pressure near the end of the stroke, where the piston actually comes to a stop, short of the end of the stroke, and bounces back a little on a cushion of high pressure air.

Due to the rapidity of compression the air is heated (hot enough to light oil vapor, should it be present).  This increase in air temperature raises its pressure to improve pellet propulsion by a useful degree, above what simple volume reduction calculations would suggest. 

This suggest that your compression ratio calculations based on residual transfer port volumes may be academic; simply because the piston never reaches the end of the cylinder while the pellet is in the barrel.  This is not to suggest that making the transfer port smaller is in anyway a bad move:  Compressing and flowing less air should be more efficient with a smaller transfer port.  Also, there are plenty of .22 airguns that work just fine with 1/8" transfer ports.

If you browsed the document at the link I posted above, you will see that TP length has by far the greatest effect on airgun efficiency.  If you could reduce that (easier said than done), the gains could be significant.

The purpose of your modifications is to reduce lock time and vibration.    Reducing the mass of the piston beyond that required to just achieve your primary goals, is likely to reduce pellet velocity rather steeply:

What hits harder; a heavy hammer swung slowly, or a hammer half as heavy moving at 1.415 times the speed (so they have the same kinetic energy)?  The lighter hammer would have only 70 % the momentum of the heavier one; and that is the key to efficient piston speed to air pressure conversion.  Don't make your piston too light...

You might argue that a lighter piston might have more energy than a heavy one, driven by the same spring.  My understanding is that steel springs are more efficient the slower they expand (less internal damping, and less affected by the springs own mass inertia).

Now, the heating effect seen with air compression and its benefits have a time component:  Faster is better.  So, if your stock spring and piston were too slow to fully benefit from adiabatic heating, then a lighter piston should gain something here...

Please let us know what you find so we may all benefit from your experience.

[subscriber]

At the risk of talking to you as thought you are new to spring air rifles, Craig; whenever someone mentions spring air rifle recoil, it makes me think they are holding it like a shotgun.  Most springers shoot more consistently with a very light hold, that if it were a shotgun would thump you solidly.

Allowing a springer to jolt backward and forwards fairly freely, virtually removes the sensation of any recoil.  All that remains is a sense of vibration.   Both accuracy and comfort improve.

A case in point are my RWS 350s in .177 and .22.  These 20 ft.lb springers weight just over 8 lb.  When held lightly there is only a sense of vibration.  When shouldered deliberately as you should a 12 gauge shotgun, these airguns produce a jolt that is surprisingly harsh; out of all proportions to the muzzle energy delivered...

Apologies for assuming you might not know this already.

[Wargruf]

I'm definitely interested in your results.
How did you sleeve the transfer port?

For the sleeve I actually used a piece of radio antenna cut to length and a touch of  locktite to seat it...
Works rather well. 

[Wargruf]

That is some good reading there Subscriber, I had read most of the document before but it’s always good to have on hand, thank you.

The piston weight I’ve selected 250g, is regarded to be slightly heavier on the optimum for 12ft/lbs so should give enough momentum for the compression stroke at this level.

Unfortunately I think the transfer port length will have to stay the same unless I invest in some more tools and the missus is already frowning at me tinkering 

When it comes to the spring I’m going to try the short and the longer one just to see how they feel.. I’ve heard that using a longer spring will help reduce bounce and also give a more subtle feel to the shot cycle without having to add too much preload and the shorter stiff spring would give a more gas ram type snap shot cycle. It’ll be interesting to see firsthand how they compare.

I know the spring in there now can throw a 15.9g pellet at 15.5ft/lbs    that’s with the preload set to just under coilbound. Soon got that back down to 11ft/lbs.

But the day is passing so I’m going out with the .177 to find some dinner.

[subscriber]

Transfer port configurations.  Includes replaceable TPs:

https://www.pyramydair.com/blog/2008/10/the-air-transfer-port-part-1/
https://www.pyramydair.com/blog/2008/10/the-air-transfer-port-part-2/
https://www.pyramydair.com/blog/2008/11/the-air-transfer-port-part-3/

[subscriber]

(Im)pertinent comment about adiabatic compression: 

https://www.gatewaytoairguns.org/GTA/index.php?topic=62010.msg594729#msg594729

Good argument, but temperature rise is time dependent.  Compression is fast, but not instant...

[Wargruf]

Well guys so far I’ve had no reply from either Tinbum tuning or Welshwilly Tuning.. 

So what I’ve done for the time being is taken the 3.3mm sleeve out and gone back to the stock 4.2mm transfer port.
I’ve also swapped the spring out for a longer slightly softer one I had knocking about and the results are good with the 250g piston.
14.5grain pellets are hitting 11ft/lbs @585fps
15.9grain are hitting at 10.6ft/lbs @548fps
21grain hit at 10.3ft/lbs @470fps(did these for fun)

These results are with the spring under its own preload of 18mm including the lightweight spring guide and top hat, with a slip washer each end.

Judging by the results I’ll add a touch more preload by fitting a longer lightweight top hat as the fring cycle is pretty decent as it is now and the recoil is more of a quick tap than a bump, which is an improvement.

If there’s anyone on here who can machine either Delrin of aluminium extensions? I’d quite happily pay for a set to be made up. I’m thinking a 40mm and a 30mm just to compare as I’ve plenty of sleeving material left over.

Anyhow I’m still hoping to get a reply from Tinbum and Welshwilly at some point just to see what they say on the spring choice front.

And thanks again for the extra info in the links, I’m almost out of research material from google and I’m re-reading Cardews Trigger to target for the umpteenth time   

[subscriber]

Thanks for the update, Craig

Glad you got where you wanted to be, with a minimum of intervention.

Please post a sketch of what you want machined with dimensions.  It is hard to volunteer, when one does not know what you are signing up for.

If you browse the Machine Shop Talk & AG Parts Machining section you may be able to identify some folks in the UK that made similar parts to those you seek.  PM appropriate people and ask if they can help:    https://www.gatewaytoairguns.org/GTA/index.php?board=15

Note that there are several sub-sections under that link; all full of good stuff.