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PostPosted: Tue Jan 13, 2015 2:11 pm 
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I enjoy reading how-to's like this. Well done, boys.

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PostPosted: Tue Jan 13, 2015 3:04 pm 
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The explanation given by Chevota, sounds fascinating,,,,However !!! if the piston face is the same diameter and the seal is the same and the spring is the same, then the lighter piston "has" to move faster simply because the lighter object at rest is easier to move when acted upon by an outside force than a heavier object. That,s just basic physics 101. I don't know the physics behind air pressure and rate of expansion and the effect of heating the air etc...etc...but it just seems to stand to reason that faster moving air should move the pellet faster also. The only thing that possibly effects the pellet speed is that the lighter piston hasn't got the "weight" for "momentum" added to the spring,s power.Thus slowing down prematurely, reducing the amount of air pressure behind the pellet before it exits the barrel.

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PostPosted: Tue Jan 13, 2015 3:29 pm 
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Probably best not to think of moving air, but more of a varying pressure/time relationship. The acceleration of the pellet is due to the force acting on it, which is pressure * area. Area is (approximately) constant, so all we really need to think about is how the pressure experienced by the pellet changes over the time the pellet travels down the barrel. It's the pressure-time graph that Chevota is referring to when he mentioned the "area under the curve", which is short-hand for saying the total force acting on the pellet during firing. You're right about a light piston moving faster than a heavy one, so we can imagine a narrow-tall pressure time graph for a light piston vs a wider-shorter graph for a heavy one. The interesting bit is how heavy and light pellets react to those differently shaped pressure pulses, and how that might effect the efficiency of transferring the stored energy in the spring to the pellet.


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PostPosted: Tue Jan 13, 2015 5:52 pm 
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The lighter piston will accelerate faster from a standing start, but it will not necessarily go faster than a heavier piston by the time each reaches the end of the stroke. A light piston accelerating fast and a heavy pellet will make a bubble of compressed air that the piston will probably bounce off due to the lighter mass of the piston. Like recoil, this all happens pretty fast. Certainly faster than our human eyes and brains can ascertain.
Interesting arguments!
Here's another example:
I rebuilt a chinese b3. I bought a rebuild kit from DL airguns and did a cylinder hone and a new seal(and a lube tune). The replacement spring was 4 coils longer and increased the cocking pressure to about 45 lbs. The rifle chrono'd 670 fps with 7.5 gr pellets. From what I've read, that's smokin' for a B3 that hasn't been stroked.
A week later, the spring failed in my Fury. I had the original spring from the B3, but it was quite a bit shorter than the Fury spring. So I put the factory spring back in the B3 and put the longer spring in the Fury.
I chrono'd the B3 with the stock spring, expecting maybe 600 fps. It chrono'd 650 fps! The rifle only lost 20 fps with the shorter and considerably softer spring. What gives? There is no question that the piston accelerated faster from a standing start with the stronger spring.
In my opinion, this fits with Chevota's hypothesis of the piston bouncing off the compressed air and also what Joe and Everhopeful said. I guess if I put the longer spring back in and added weight to the piston, the velocity should increase over 670 fps. No, I'm not gonna do it. Both guns are now shooting way too smooth to mess with.

Any other opinions? There must be some physics guys that can explain what is happening in layman's terms.

This is all good stuff. I learn something every time I go on this site.

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PostPosted: Tue Jan 13, 2015 7:40 pm 
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Not a physics guy here, but going on Chevota's comments and wanting to smooth out performance of my newly rebuilt QB57 I thought to try adding a bit of mass to the piston today. The velocity wasn't the focus, as I'm more interested in damping the harsh noise coming from the cylinder which doubles as a cheek rest on this bullpup. It seemed sensible to me that if I added a bit of mass to the piston this might reduce bounce, and my initial result seems to have borne that out. A small but distinct drop in the *ouch* factor right next to my ear, and a proportional reduction in duration of that sound. It's almost as brief now as bouncing a hammer off another hammer, and of similar character.

I had preloaded the spring about 14mm from the spring guide side yesterday and this seemed to help a little with getting more consistent velocity out of my JSB Heavy pellets (lighter pellets were just showing too wide a jump from shot to shot and the noise was unpleasantly loud), so I kept about 8mm of that preload when shifting to the front of the spring, compromising. I used one 4mm thick thin walled nut at the front, then a steel part left over from a doublebass endpin, a sort of heavy tube end cap with a 4mm thick rounded head and the rest, about 10mm long, pressing snug into the spring. Combined weight 25 grams, or about 14% of the piston's mass before weighting. The rounded head nestles nicely into the nut so it's stable as a spring base. I'd also ground off a few millimetres of the spring to square the ends, so net preload compared to factory condition is only about 5mm.

The result was as mentioned, a drop in audible impact volume and duration, making for a slightly more pleasant experience. In terms of velocity, this increased by 5.4% with about a 2% tightening up of shot-to-shot consistency, or ES I guess it's called. Not a huge change in any sense, but it works slightly better so I'm happy.

Next up will be laminating the cylinder with some sort of sound absorbing material where it's outside the stock, probably building up some damping inside as well, and wrapping the cocking arm with similar to seal up when locked into place. That ought to make it considerably quieter on the ol' eardrum and make a softer place to put my face besides. I'm leaning towards thin layers built up, perhaps alternating layers of a couple of different materials to more effectively damp vibration.

Anyway, thanks or this discussion, it's most interesting to see springer tuning come up when I see so much PCP tuning stuff around. These cheap springers can be a lot of fun, especially with the lack of pumping. Nice to be able to optimize them at least a little. Perhaps sometime I'll try machining a brass button to fill the end of the cylinder with similar or less preload but more mass and see how that works out...


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PostPosted: Tue Jan 13, 2015 9:40 pm 
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Hello Gerard:
I had some pond liner left over from a previous project. It is like thick inner tube and is available at home depot, I think. It would make a good sound deadener on the stock of your qb57. I have also used it to "rubber bed" a synthetic stock by placing a layer of it wherever the action contacts the stock, then tightening the screws. I don't think I'd try it on a wood stock, it might crack. The synthetic stocks I have tried it on just flexed out of the way. It really made the shot cycle quieter. I placed two thicknesses of it in front of the tophat in my Fury last time I had it apart thinking that it would absorb some of the shock and vibration of the firing cycle. I did some other tuning at the same time including shrink tubing the spring guide so I can't isolate the results of the rubber dampening in front of the top hat. Shoots with a quiet "thoomp" now.
Talk about lipstick on a pig...

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PostPosted: Tue Jan 13, 2015 9:53 pm 
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Dang....things are moving along here.

My interpretation from what I have read (here and Trigger to Muzzle) so far goes something like this;
It seems inertia plays a vital role in the cycle. When the piston is released, compression occurs until the pellet is propelled. During this stage, the piston is momentarily slowed down until inertia is overcome, at which time, the compression is suddenly decreased. Only now, the piston has to "catch up" to the pellet as the pellet is moving faster than the piston can re-compress. I wonder if during this stage, the compression "flatlines" for a millisecond, then the pellet is hit again by a second compression wave. If this is correct, one could assume that a shorter barrel would be more efficient than a longer one. That is to say, to have the pellet exit the barrel at the peak of the initial compression stage. I wish I had a few spare barrels to experiment with.

Again, this is just my understanding....I may be way off base.

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PostPosted: Tue Jan 13, 2015 10:32 pm 
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Handyguy wrote:
Dang....things are moving along here.

My interpretation from what I have read (here and Trigger to Muzzle) so far goes something like this;
It seems inertia plays a vital role in the cycle. When the piston is released, compression occurs until the pellet is propelled. During this stage, the piston is momentarily slowed down until inertia is overcome, at which time, the compression is suddenly decreased. Only now, the piston has to "catch up" to the pellet as the pellet is moving faster than the piston can re-compress. I wonder if during this stage, the compression "flatlines" for a millisecond, then the pellet is hit again by a second compression wave. If this is correct, one could assume that a shorter barrel would be more efficient than a longer one. That is to say, to have the pellet exit the barrel at the peak of the initial compression stage. I wish I had a few spare barrels to experiment with.

Again, this is just my understanding....I may be way off base.


As to bbl legnth to piston cycle dynamics. The experiment was done years ago and depending upon the pellet weight and sweep area 8.5 to 11" of bbl length was only required the rest became unneccesary friction.

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PostPosted: Tue Jan 13, 2015 11:01 pm 
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Whitewolf wrote:
Handyguy wrote:
Dang....things are moving along here.

My interpretation from what I have read (here and Trigger to Muzzle) so far goes something like this;
It seems inertia plays a vital role in the cycle. When the piston is released, compression occurs until the pellet is propelled. During this stage, the piston is momentarily slowed down until inertia is overcome, at which time, the compression is suddenly decreased. Only now, the piston has to "catch up" to the pellet as the pellet is moving faster than the piston can re-compress. I wonder if during this stage, the compression "flatlines" for a millisecond, then the pellet is hit again by a second compression wave. If this is correct, one could assume that a shorter barrel would be more efficient than a longer one. That is to say, to have the pellet exit the barrel at the peak of the initial compression stage. I wish I had a few spare barrels to experiment with.

Again, this is just my understanding....I may be way off base.


As to bbl legnth to piston cycle dynamics. The experiment was done years ago and depending upon the pellet weight and sweep area 8.5 to 11" of bbl length was only required the rest became unneccesary friction.


Interesting. Thanks Kim.

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PostPosted: Wed Jan 14, 2015 12:14 am 
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I'll second that, very interesting. Have to do some searching and read up on that experiment, see how it applies to the QB57 with the given short stroke. Of course replacement barrels are easy to come by for this one at least:
http://www.archerairguns.com/QB57-Barre ... barrel.htm
Well, easy if you have $26 plus shipping and live in the USA. Which I don't. But I suppose some friend or other could receive it and pass it along to me later, if I felt like experimenting with chopping off an inch at a time and testing until velocity started to drop. I read somewhere that a similar experiment had been done with a .22"LR, starting at a full length rifle barrel then gradually chopping off and testing. It turned out 2" was a bit too short for accuracy, but 4" or so seemed plenty. Velocity dropped a little which each cut to the barrel, but even at 4" or 4.5" it seemed sufficient for small game use. Of course a spring and piston is considerably different from rimfire cartridges, but testing to find the optimum barrel length for a given power plant and pellet weight/diameter seems a valid pursuit for the curious.


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PostPosted: Wed Jan 14, 2015 6:50 am 
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I wonder if changes to the transfer port on a springer has any significant effect ?

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PostPosted: Wed Jan 14, 2015 9:34 am 
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joe hickey wrote:
I wonder if changes to the transfer port on a springer has any significant effect ?


I can attest to this with authority; it does and not for the better. I have a junk springer that I ventured to modify with your question in mind. I bored out the transfer port by 1/32" and the velocity dropped by a third. This confirmed the theories regarding compression. Opening up the port increased the airflow, but significantly reduced the compression. I am now in the process of making a sleeve to reduce the TP bore, only this time, I'm going to make it about 1/32" smaller than factory.

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PostPosted: Wed Jan 14, 2015 11:14 am 
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Handyguy wrote:
joe hickey wrote:
I wonder if changes to the transfer port on a springer has any significant effect ?


I can attest to this with authority; it does and not for the better. I have a junk springer that I ventured to modify with your question in mind. I bored out the transfer port by 1/32" and the velocity dropped by a third. This confirmed the theories regarding compression. Opening up the port increased the airflow, but significantly reduced the compression. I am now in the process of making a sleeve to reduce the TP bore, only this time, I'm going to make it about 1/32" smaller than factory.


Please keep us updated as to your progress. I have never tried any transfer port tuning. Based on some internet reading, your results so far mirror what pretty much everyone else has found.

Wait a minute...I lied. I have done some transfer port work. Just remembered that a friend gave me a Relum (FEG) Taurus breakbarrel in .177 with a bb stuck firmly half way down the transfer port. I bashed it through with a steel punch (what else could I do?) and did a lube tune while it was apart. The transfer port looks like 10 miles of bad road inside, but the gun shoots well at 550 fps. Right on what it should shoot for a single spring. The original had two mainsprings, one inside the other with no spring guide. This model has been converted to a single spring with a spring guide. Very accurate gun.

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PostPosted: Wed Jan 14, 2015 11:41 am 
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Handyguy wrote:
joe hickey wrote:
I wonder if changes to the transfer port on a springer has any significant effect ?


I can attest to this with authority; it does and not for the better.

And of course the part where the air goes through the transfer port invalidates what I said earlier about thinking in terms of pressure and ignoring the flow. Oh well, I guess each approach has it's applicability and limitations, and you probably need to consider both to get a good handle on system behaviour.


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PostPosted: Wed Jan 14, 2015 11:52 am 
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The eyes glazing over, boys, is not a problem, but the head-heat is making smoke come out my ears. :rolleyes:

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