Springer pressure at end of stroke

[rsterne]

Unless I am mistaken, a piston with a complex shape, as the stepped one in Reply #23 above.... with valving the allows the air to pass in either direction between the small and large diameters.... will not provide any boost in pressure over a flat, single diameter piston of the same bore and stroke.... The shape of the front of the piston should not matter, if the compression chamber in the front of the tube matches that profile, and the headspace is the same.... at least that is what I see happening....

Think of a flat piston and flat end on the compression tube.... compared to a domed piston and matching concave end on the front of the tube.... If both have (for example) a 0.050" space, and a 5" stoke, they should have a compression ratio of 100:1.... and produce about 1500 psi (assuming a strong enough spring to get there)…. I think the same principle applies to a stepped piston and reverse step in the front of the compression tube, and any valving would only serve to equalize the pressure difference, if there was any.... That's what I see, anyways....

Now subscriber's progressive piston with the inner (or outer) portion moving separately through its own momentum may be a different story.... but if the portions of the piston are rigid (ie one piece), I can't see a way to boost the pressure....

Bob

[subscriber]

Getting back to the "deadblow hammer" piston design.  The question is, how much would one want to get the piston to dwell at the end of its stroke, that now represents lost performance?  This would effect the optimal open space in the shot tube (linear distance is the important factor, at nominal piston deceleration).  Also, the ratio of piston mass that is made up of shot would factor into holding peak pressure longer, without reducing the peak pressure significantly.

Looking at figure number 14 of that spring air rifle study, the answer appears to be just under 2/1000 second:
https://www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns

Obviously, this is a very specific case, but it gives us some idea.  Using some very fuzzy math where I "divide by the number I first thought of", I think that free play in the shot column (with all shot against the back wall, then all shot against the front wall) of 1/4 to 1/2" would be about right... 

I still think that the shot weight should be 1/3 to 1/2 the total piston weight, with less than 1/4 probably not being very useful.  Considering the space inside the spring for shot, tungsten is starting to look like the best solution.  That, after perhaps machining off 1/3 to 1/2 of the original piston weight...

Based on the quote below from section 6.4 of the linked study, simply adding piston weight may help boost pellet velocity too.  So, just adding weight in the form of shot, without reducing the original piston weight may be beneficial.  However, the gain from a simple mass increase is small:   

Computations show that, at the margin, an increment of one gram in piston weight leads to an increment of about 0.047 Joules in power output for the 0.547 g pellet

There are reasons why simply adding lots of mass to the piston may not be ideal in practice, such as a slower harsher shot cycle that increases hold sensitivity; boosting velocity at the expense of practical accuracy.  As such, your preferred embodiment would be to keep the piston mass almost original, except that a large portion of that mass is comprised of movably contained metal shot.

[rsterne]

Generally speaking, with no other changes, increasing the piston mass optimizes a springer for a heavier pellet.... The heavier pellet takes longer to accelerate, giving more time for the air cushion to arrest the higher momentum of the heavier piston.... Heavy pistons and light pellets tend to make for a very "harsh" shot cycle, with the gun jumping forward as the piston collides with the end of the compression chamber instead of bouncing off a cushion of air.... More power, maybe, but often at the expense of less accuracy.... The same can apply if you drill out the transfer port, it tends to make the gun harsher with light pellets.... Well made springers have a delicate balance between all these factors, in many cases quite well worked out by the manufacturer.... particularly in the more expensive, higher quality springers....

Bob

[subscriber]

Well made springers have a delicate balance between all these factors, in many cases quite well worked out by the manufacturer.... particularly in the more expensive, higher quality springers....

So, if one is going to try to improve over standard springer design, start with a cheap airgun that won't make you cry if you turn it into scrap...

I wonder if a piston of the same weight, but with deadblow debouncing would be any more likely to crash into the end of the cylinder.  My gut says, no.  I predict it would behave a little better, regardless of pellet weight (within reason). Assuming the execution is appropriate to work with the rest of the system...

Damping piston bounce may even increase spring life as an added bonus.

[MartyMcFly]

I've been scouring website deals for just such a springer. It will likely be a Hatsan or a Gamo, one of those magnum ones.

Thanks for that link to QVTom's post, I wish I had his skills!

As for piston slam, I agree that as long as you keep the entire piston assembly and BB shot filler at the same weight as the unmodified piston then it shouldn't be a problem. Dialing it in will take some tries, but it should be ok.

Well made springers have a delicate balance between all these factors, in many cases quite well worked out by the manufacturer.... particularly in the more expensive, higher quality springers....

So, if one is going to try to improve over standard springer design, start with a cheap airgun that won't make you cry if you turn it into scrap...

I wonder if a piston of the same weight, but with deadblow debouncing would be any more likely to crash into the end of the cylinder.  My gut says, no.  I predict it would behave a little better, regardless of pellet weight (within reason). Assuming the execution is appropriate to work with the rest of the system...

Damping piston bounce may even increase spring life as an added bonus.

[MartyMcFly]

subscriber, have you thought about the variability in performance of the dead hammer design depending on the angle that the rifle is pointed? Would the initial position of the steel shot, for example when the rifle is pointed downward, create large variation in either recoil or velocity?

Also, would placement of the steel shot compartment at the end versus at the front of the piston assembly have different performance characteristics? My gut tells me that the forces involved would be the same, but then again pulling something forward versus pushing it may have an impact on recoil characteristics. Would having the steel shot near the back of the piston have less tendency for the rifle to “tip” forward as its center of mass changes?

-Marty

[subscriber]

In theory, pointing the rifle down, up or level would change the way the shot is "collected" as the piston accelerates initially.  Exactly how much variability that might introduce is something to be determined experimentally.

Unless the "free space" in the shot column is "huge", I don't think the starting position of the shot is going to cause any problems.   Here is why:

Practically speaking, most shots are fired with the barrel not many degrees off from horizontal; either up or down.  Also, the shot would end up "finding its own level" like a liquid; such that at rest the free space is at the top of the tube, rather than at the front or rear.

If you are shooting down from a tree stand, then all your shots are from that position. So, even if it shifts the POI from that when shooting near level, a little, you have to account for range and angle anyway when you shoot steeply "down" or "up".

When selecting a candidate rifle to modify, I suggest one without an integral spring guide that is part of the piston, that also contains the sear notch.  This style of piston occupies the space you want to place the shot tube...

Choose piston #2 below; not #1 below:

[subscriber]

I think the tendency for the rifle to "tip forward", if the shot is concentrated near the front of the piston will be hard to measure, let alone feel:

Can you tell if your spring air rifle is cocked, versus fired; based on a change in its center of gravity?  That involves a 4" shift in the position of 100 % of the piston mass and half that of the spring.  If the steel shot makes up half the piston mass, and it has shifted 1/2" forward, I don't think anyone will notice...

[MartyMcFly]

Yes, but depending on the location of the pellet in the barrel, as the piston comes to the end of the stroke, it may have an impact on accuracy. Probably minimal, but something that is worth testing.

Another thing I’d like to test is if the diameter of the steel shot is important. I dont’t have any Mercury on hand to make a comparison to steel shot, but I would guess that the more it acts like a fluid the faster it would self level against the back of the piston at the start of the stroke. Now I’m just nitpicking, but as they say the whole is sometimes greater than the sum of its parts.

-Marty

I think the tendency for the rifle to "tip forward", if the shot is concentrated near the front of the piston will be hard to measure, let alone feel:

Can you tell if your spring air rifle is cocked, versus fired; based on a change in its center of gravity?  That involves a 4" shift in the position of 100 % of the piston mass and half that of the spring.  If the steel shot makes up half the piston mass, and it has shifted 1/2" forward, I don't think anyone will notice...

[MartyMcFly]

I may have to buy two identical rifles, just so I can get an objective comparison of before and after. 

[subscriber]

Due to the peak acceleration at max spring compression, the steel shot would collect completely at the rear of the piston within a gnat's whisker of initial piston travel.  Only when the piston starts to see negative acceleration does the shot column start moving forwards.  By the time the piston stops, there would be no difference in shot position or density to affect the pellet leaving the barrel.

In fact; if one could write a spec for the shot string settling inside the piston; it needs to completely stack towards the rear and front of the piston, at the start and end of the piston travel respectively; within 0.002 second - the same time interval for both events.

The reason to use shot is because each little pellet transmits its momentum in a continuous stream (for 0.002 seconds) of small sequential incremental impacts.  Also, the sliding of pellets over each other as they "pack" into peak density causes the action to be damped, as if the shot column is a viscous fluid.   

Using an actual "thin" liquid such as mercury would probably result in the equivalent of water hammer; where the entire slug of fluid crashes into the front of the piston almost simultaneously.  Yes, the air in front of the mercury would have to travel through the liquid, and that would break up the stream to some degree. 

This venting of air would not be as predictable or repeatable through mercury, compared to steel shot; because of all the discreet spaces between shot pellets that never collapse completely.

If you made some experimental hammers where you varied the size of the shot and the size of the free space, then whacked the hammers into a steel plate and observed the lack of bounce, that would be useful.  While you are at it, try a mercury or water hammer.

[subscriber]

Start with the same size shot as is used in dead blow hammers....

[MartyMcFly]

Any good reason to consider a conical shape for the back-end where the steel shot gathers during acceleration?

Start with the same size shot as is used in dead blow hammers....

[subscriber]

Any good reason to consider a conical shape for the back-end where the steel shot gathers during acceleration?

If some deadblow hammers have cones at the back (how does one tell the back end when the hammer head is symmetrical on the outside?), this may be to soften the reaction on you hand, as the pellets collect at that end of the hammer tube; just after you start the swing.  Another reason may be to reduce pellet bounce and separation on the very gentle acceleration humans are capable of at the start of the swing; compared to the abrupt stop at the end when you hit something stiff with the hammer.

Making the front end of the airgun piston shot tube conical may be a way of increasing the dwell, if that is required.   Somehow, I think that might "oversolve" the bouncing problem, and reduce the force of the blow.  You don't want to reduce the force of the piston slamming into the air column (if you can help it); you just want to damp the tendency of the piston to bounce on the air spring in front of it, long enough to clear the pellet from the muzzle.

[MartyMcFly]

I spent a part of today looking at suitable platforms for this dead blow hammer experiment. Aside from the Stoeger X20, any other suggestions for suitable cheap springers to experiment on? Easy disassembly are a plus, larger piston diameter is probably good too because it would make placing a another cylinder inside it easier. Since we are trying to see if piston bounce is reduced any known models with this problem would be good candidates as well. 

Thanks,
Marty

[subscriber]

All spring airguns "suffer" from piston bounce.  One might even call it a feature, as it reduces muzzle pressure and quiets the report.  Yes, at the expense of velocity...

Probably any airgun sold with a "gas ram" would have the piston ID open.  As many spring guns have identical versions with gas rams, converting back to a spring would seem feasible.  To get appropriate spring, shop here:  https://www.pyramydair.com/search-results-ext?keyword=main+spring&sid=1375A617A415&N=0&Ntk=primary&q=main+spring&cx=002970863286801882398:jlcminxfwdw&cof=FORID:11;NB:1&saSearch


This Benajamin Trail would seem a candidate with an open piston ID.  :   http://www.airguntoys.com/NP-GRS_files/GRS-NP_PrintPage1.htm





https://www.pyramydair.com/blog/2013/01/ive-got-gas-part-1/



https://www.airgununiverse.net/wp/gamo-shadow-teardown-and-tune/




RWS Diana 350 with appropriate piston:




Found these piston images and articles by searching google images for gamo piston seal replacement.  If you have instructions on how to get the piston out, that would seem logically to include images of the piston:  https://www.google.com/search?rlz=1C1NDCM_enUS793US793&biw=1524&bih=712&tbm=isch&sa=1&ei=fPLYW5nULsSS0wLLhJSICg&q=gamo+piston+seal+replacement&oq=gamo+piston+seal+replacement&gs_l=img.3..0i24k1.234970.237256.0.238127.6.5.1.0.0.0.112.412.4j1.5.0....0...1c.1.64.img..0.1.112....0.RhDgFVeusWI#imgrc=jmyoXa9r1ydffM:

Change "gamo" for any brand name.  Using "airgun" or "air rifle" yielded too many unrelated images...


You may want to invest in something like this too:  https://www.pyramydair.com/product/air-venturi-rail-lock-spring-compressor?a=7764

[subscriber]

Another way of looking at the piston design of any air rifle sold by Pyramyd is to download the user manual or schematics.

For instance, here is the Trail: https://www.pyramydair.com/product/benjamin-trail-np-xl-air-rifle?m=2052
Here is the schematic, found by scrolling down on the right side of the "description" tab of the above page: https://www.pyramydair.com/air-gunsairsoft-schematics

[subscriber]

Schematics for a bunch of airgun brands:  https://www.pyramydair.com/air-gunsairsoft-schematics

[subscriber]

Low cost gas ram rifles:  https://www.pyramydair.com/air-guns?orderBy=price_asc

Pick one with a high customer rating, because that probably means the rest of the parts are not complete rubbish.  Also, find discussions about taking them apart and choose one that is easy to work on...

[MartyMcFly]

I've ordered one of the Hatsan models with a gas ram and I'll be ordering a spring to go with it soon. I've heard that converting the gas ram to spring is easier in the Hatsan rifles than some others. If not, oh well, I'll just have another rifle 

In the meantime I've also done some more reading on the subject and came upon the Theoben line of rifles. These sweet English beauties make some of the German Fräulein look quaint. A few were sold in the US under the Beeman name as the Crowmagnum models. Getting back on topic, one of Tom Gaylord's articles mentions that the Theoben Eliminator had a floating inertia piston design coupled with some kind of value. Has anyone seen the internals of these guns or a diagram?

-Marty