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  Re: Airgun tuning
« Reply #15 on: May 27, 2008, 20:40:11 PM » by Tuner
Tuning - This can be a strange word indeed! -  Tuning began in earnest with the spring air rifle.   There is so much to be done to a factory mass produced gun.   This is because, the spring air rifle, is really just a machine that generates compression with a barrel at the end to project a pellet down range as accurately as possible!!!  It can be a break barrel, an underlever, a side lever, a telescopic arrangement, like many early air pistols, and more.
The factory spring will have to be changed. (Not because the factory cannot fit these springs at the time of manufacture, but they are  concerned about the final price of the product !!!!!)   You can choose a strong spring, or a 12 ft/lb spring.   The 'strong' (stiffer) spring will either have wider spaced coils (longer), thicker wire, and less coils, or smaller diameter coils.   But the most important thing is that it is made from Swedish Silicon Chrome Steel, shot peened and stress relieved. The compressed length must not be longer than the standard spring, or the gun will not cock.   The best springs also have a baked on Moly coating so no lubrication whatsoever is needed!!!   These springs can be left cocked for weeks and never loose power!  They will cost about 5 times more than a standard factory spring.
This is just the begining - the piston seal will have to be changed - the transfer port 'air flowed' and reamed to an exact formula worked out by experts in the subject.   Most guns will have a different diameter transfer port, depending on many things! - the cylinder will have to be polished and honed to a mirror finish.
The object is to have a gun requiring less effort to cock and  producing more power!!!!  -  seems impossible, but it is not!  -  This is a taste!!!! more will follow for those interested in the subject.    But be warned, no one without any engineering background should ever tamper with an airgun - spring or PCP!!!!  - Danny Gatt
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  Re: Airgun tuning
« Reply #16 on: May 28, 2008, 20:29:45 PM » by Tuner
Tuner - Have you ever heard the words - piston bounce - ?     I am certain, that most airgunners are not familiar with this 'mystic' happining in the cylinder of their guns every time a shot is fired!
This is the most  important matter that has to be tuned to a fine degree - even to a perfect degree - if possible!
Fire a spring gun, and if there is felt vibration, there is a 90% chance that the piston head is 'crashing' into the end of the cylinder!!!! -  a sure way to uncomfortable shooting and to the swift destruction of the airgun!   The screws holding it to the stock, keep comming loose - the whole thing is comming apart!!!
If, on the other hand, when the gun is fired, no vibration at all is felt, but a 'slight' recoil (experience will help to recognise this behaviour) is detected, then there is a 90% chance that you have 'piston bounce' !   -   all the effort required in cocking strong springs plus all other extras fitted,will be of no avail, as the 'peak compression' is being wasted!!!!!!!!!     Dany Gatt  -   dannygat@maltanet.net
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  Re: Airgun tuning
« Reply #17 on: May 29, 2008, 08:45:58 AM » by RB
IMHO the only way that a piston would crash into the end of the cylinder, would be if there is no pellet in the breech, else the breech seals or the piston seals are very far gone. The vibration that is felt with most standard springers is generally spring vibration, which is very easily cured with a tighter spring guide, even if at a very slight loss of power.

Piston bounce - the Cardews discovered that this always happens as long as there is a pellet in the breech. This of course does not refer to the very violent and great rearwards movement that happens when a springer diesels - i.e. when there is too much lube ahead of the piston and much of this burns - but to the slight rearwards motion of the piston that occurs as the piston almost reaches the cylinder face. They discovered that when a springer is fired, the piston very obviously initially shoots forward, then at a point very close to the cylinder face because the temperature is elevated and a very tiny amount of lube burns, the air ahead of the piston reaches pressures high enough to completely stop the piston and push it backwards a fraction of an inch. Somewhere around that time of course the pellet starts to move, thus the pressure is reduced, and the piston comes to rest at the cylinder face.

They did their utmost to prevent the rearwards movement by designing a "non-return" rig so the piston could only move forwards, but quickly discovered that it was in practical terms impossible for their already too heavy setup was actually being damaged.

In practical terms the easiest way to tune for efficency in this respect is simply to experiment with different pellets, and possibly consider the design of the breech. The main concern here being how much resistance the pellet offers to initial travel.

If the pellet is sized and is a loose fit, this will depart before peak pressure is reached, and power is lost. If it is too tight, the piston will bounce more, and again power is lost. However the bias is more towards a tight pellet than a loose one, which is why sizing, rather than increasing power as the vendors of sizers would have us imagine, is a bad idea with a springer.

RB
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  Re: Airgun tuning
« Reply #18 on: May 29, 2008, 09:59:01 AM » by dvd
Hi RB, Tuner,
I agree with what you said.
The whole setup is so finely balanced that when one comes close to nearly eliminating violent piston bounce, one important component must stay constant and that is how the pellet is placed in the barrel.
Probe seated pellets generally eliminate this variable.
Assuming compression, lubrication, piston weight,  spring power and air flow is fine, it is pellet fit in the breech that determines the behaviour of the airgun on discharge. Sometimes changing  pellet weight and/or dimensions is enough to undo all the careful measurements to eliminate piston bounce.
Try simply pushing a pellet further in or further out by just a millimetre in the breech and watch your impact point shift.
This is more noticeable in springers whose air blast is more incremental compared to PCP's and as such preserve the pellet skirts better.
Since optimum compression is achieved by the pellet fit in the breech and is not mechanically controlled as in an internal combustion engine, removing as many variables as possible is the key to consistency and an accurate airgun.  Smiley
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  Re: Airgun tuning
« Reply #19 on: May 29, 2008, 10:23:25 AM » by RB
This is more noticeable in springers whose air blast is more incremental compared to PCP's and as such preserve the pellet skirts better.

Not sure if I misunderstood you, but a springer is far harder on pellets than a PCP, and occasionally prone to blowing out the skirts when reaching the practical limits of power. The peak pressure attained behind a pellet in a springer is considerably higher than that attained in a PCP. I think I read somewhere that it is in the order of around 1,600 psi. However, it is a short lived peak which is why a springer is fine with a short barrel. The push from a PCP is lower pressure, more gradual, and utilises a greater volume of air, which is why a PCP is best with a long barrel, even to ridiculous extents.

Sure, I do know that a PCP is generally charged to 200 bar which is a lot higher than 1,600 psi, but it is the pressure that is actually developed behind the pellet that counts, and due to the fact that the airflow is (relatively of course) rather slow/restricted from the exhaust valve to the breech, plus of course the actual compressed volume of air released has expanded, peak pressure never gets anywhere near 200 bar or even half that.

Reasons of flow and consequently peak pressure behind the pellet is why PCP's filled with helium produce ridiculously high power levels from the same cylinder fill pressure.

RB
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  Re: Airgun tuning
« Reply #20 on: May 29, 2008, 20:32:44 PM » by Tuner
Tuner -  Dear RB, I have to admit I am very pleasantly surprised that the works of the'Cardews' is read and appreciated in Malta.   My compliments!.
However, there are instances when the piston 'crashes' into the cylinder end, of course, with the pellet in the barrel!   Only a fool would dream of shooting a springer without a pellet!!!!! -
You are quoting from Cardew's book, Airgun - From Trigger to Muzzel' - page 26.   This is true, but here, Cardew was speaking of a perefectly tuned airgun of low to medium power, where , for experimentation sake, he could have used a rather heavy pellet for a springer, because, as correctly stated , the weight of the pellet and fit in barrel, has great influence on piston bounce!!
But Gerald Cardew, a dear friend, has also written many 'Papers' on specific subjects, and this subject is treated in the 'four phases' of operation of an airgun at the moment of firing, namely;   a)  The Popgun phase   -  very low power,   b)   The 'Combustion Phase'  -  where only a small portion of lubricant is burned adding to the power of the shot (the ideal arrangement),   c)   The 'Detonation Phase   -   where too much lubricant explodes, giving an extraordinary power to the shot and harming the airgun significabtly,    d)   Finally we return to another Popgun Phase, where only powder lubricants or lubricants with an extreemly high flashpoint are used and the gun just uses its air power.
Cardew, in page 26 of his book, was not referring to a gun of small swept volume, where an VERY strong spring is fitted and a light pellet used!   This arrangement would start pushing out the pellet up the barrel, even before peak compression has been generated, and will bundle out, (as Cardew puts it) the pellet, the grease and all out of the barrel in nearly Popgun Phase performance!!!!!   I have tried this many times and proved it myself!
I had not intended to enter into this subject at this level, because I meant my writings as passtime for beginners, but, you 'shot down' my subject 'Piston Bounce' too harshly not to merit a reply!
Good shooting friend.   Danny Gatt
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  Re: Airgun tuning
« Reply #21 on: May 29, 2008, 20:38:44 PM » by Tuner
Tuner -  I excuse myself for the wrong spelling of 'Muzzle', but I admit I got carried away by the subject!!! Cry  -   Danny Gatt
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  Re: Airgun tuning
« Reply #22 on: May 29, 2008, 20:40:43 PM » by Tuner
Tuner  -  For the many spelling mistakes!!!!!!!!!!    Im really gonbe now! Embarrassed Embarrassed Embarrassed Embarrassed    Dannny Gatt
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  Re: Airgun tuning
« Reply #23 on: May 30, 2008, 07:42:29 AM » by dvd
Hi RB,
you are perfectly right,
I meant to say that the pressure release into the barrel in a PCP is more incremental than in a springer. OOPS!!
Your example of shorter springer barrels achieving comparable velocities to longer PCP barrels is spot on.
My mistake and good of you to point it out.

The typical pressure for dieseling to start is around 500 to 700 psi and depends on the volatility and quantity of the lubricant used.The rate of compression leads to a temperature rise enough to ignite any oils present. This raises the pressure drastically and pellet inertia and friction usually results in skirt deformation.
Once combustion starts, it can be quite disastrous and wasteful if not kept within certain limits.
Balancing  these and other factors so that they contribute to velocity and accuracy is what tuning is all about.

In a PCP it is the reverse of what happens in a springer, ie, the already compressed air which is at ambient temperature is released and this results in a drop in pressure and temperature, thereby eliminating any possibility of dieseling in a PCP.
 I'll try to keep my contributions on the subject as plain as possible for the benefit of everyone, as you rightly pointed out Tuner,
In my opinion it is wise to appreciate airguns for what they are and strive for accuracy before power.

Perhaps Tuner, you can share your knowledge about diablo pellet design and why they are shaped contrary to perceived ballistic wisdom and therefore doomed to sub sonic performance only.

 Best regards to all.
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  Re: Airgun tuning
« Reply #24 on: May 30, 2008, 10:08:56 AM » by RB
Tuner, reading your post, I have no idea which part of mine you do not agree with??

RB
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  Re: Airgun tuning
« Reply #25 on: May 30, 2008, 20:41:22 PM » by Tuner
Tuner - Sorry DVD, but I cannot even start to explain the use of the diabolo shape pellet, because you already state dogmatically that this pellet, because of its design, is doomed to sub sonic levels!!!!!   Certainly not, ultrasonic levels are reached in many guns (Springers or PCP) with rediculous ease!!!
You have already reached your own conclusions, so it will be useless for me to persist! -
Happy Shooting and may I dare say (tuning?)   -   Danny Gatt
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  Re: Airgun tuning
« Reply #26 on: May 30, 2008, 20:45:47 PM » by Tuner
Tuner  -  Dear RB, I do not agree at all with the content in the first sentence of your first paragraph where you state that a piston cannot crash into the end of the cylinder if there is a pellet in the barrel!   As I explained, it certainly can given certain (common) circunstances!!!!   - Danny Gatt
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  Re: Airgun tuning
« Reply #27 on: May 31, 2008, 08:23:09 AM » by RB
I think that DVD means to say that the design is far from suitable for supersonic (not ultrasonic, I think you have been doing too much cleaning  Grin  Grin) velocities.

Not that it is not possible for a diabolo to REACH supersonic velocities.

While I have in the past experienced supersonic velocities from a springer, and "impossible" is certainly not a word I am happy to use, I do not consider it "easy" to go SS with a PCP under normal circumstances, which does mean using air as a fill gas.

Again as far as piston crash goes I would wish for an explanation as to under which "normal" circumstances a properly functioning air rifle would do this. I had specifically excluded guns with poor or no breech or piston seals, etc, so this is not what it is about. I would also like to know how you did actually determine that the vibration you felt in certain guns was a result of piston crash and not anything else.

We learn something new every day and I have no problem with standing corrected.

RB
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  Re: Airgun tuning
« Reply #28 on: May 31, 2008, 08:54:42 AM » by dvd
Hi Tuner, RB,
yes,  not saying that it can not be pushed beyond supersonic speeds, but perhaps pushing a diablo shape beyond 1200fps is asking too much of its shuttlcock shape and accuracy suffers. What's the use of power without accuracy?

The reason I say this is because the pellet has to go through not one but two unstable phases of crossing the sonic barrier, first when it is still accelerating and second, when the air resistance slows it down again to the critical speed. If the target happens to fall beyond these two distances, accuracy is bound to be poor.
In an ideal situation, if supersonic speed is exceeded, it should be maintained at least until the target is reached.

I have experimented a fair bit and so far I think that about 850fps for a normal 22 diablo pellet is optimum between accuracy and power.On the other hand 580fps for an unaerodynamic flathead 177 pellet, delivers amazing accuracy but obviously little power. I think that the higher the speed goes, the less stable diablos become. Weight, its distribution, rear drag and a hollow tail complicate things no end as velocity rises.
 
It is true what you say that I have my own ideas on this subject but surely different perspectives are always healthy for a better understanding and will perhaps open up new thoughts on this subject. I know a fair bit but I still discover new things when I discuss such things with like minded folk.
There are so many variables in this game that one small change upsets the whole lot. I am interested in determining those constants that make airgun accuracy such an elusive creature and analysing the projectile shape and performance is only one part of the equation.
 
Happy shooting chaps Smiley
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  Re: Airgun tuning
« Reply #29 on: May 31, 2008, 09:00:51 AM » by RB
DVD, stability or whatever will not be an issue in phase one - the pellet will still be in the barrel  Grin - unless your pellets are rocket boosted of course  Grin   Grin

RB
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