At what point does the pellet exit the barrel?

[Mark 611]

Scott, your reading my mind!

[Yogi]

Interesting reading here.
  At first I wanted to measure the barrel lengths on my Springers to see if the more accurate ones had longer or shorter barrels. HW-98, R-10, and R-9, FWB 124....
But then it dawned on me that the compression tube length plays into the piston travel distance. So that would determine when the recoil changes direction and the amount it travels in conjunction with the barrel length.
  Way too much math this early in the morning. 

Scott,

You forgot the size and shape, and location, of the transfer port as well.....

-Y

[UnderPressure]

Interesting reading here.
  At first I wanted to measure the barrel lengths on my Springers to see if the more accurate ones had longer or shorter barrels. HW-98, R-10, and R-9, FWB 124....
But then it dawned on me that the compression tube length plays into the piston travel distance. So that would determine when the recoil changes direction and the amount it travels in conjunction with the barrel length.
  Way too much math this early in the morning. 

Scott,

You forgot the size and shape, and location, of the transfer port as well.....

-Y

Not only that but the strength of the spring (plus whether it is a linear rate spring or a progressive rate spring), piston weight, friction of projectile within barrel, projectile caliber and weight, barrel length.... just to make a few.

Of particular interest is the strength of the spring compared to the piston weight. Logic follows kinetic energy of the piston during compression will increase with increasing piston weight up until a point and then further increasing piston weight results in decreasing kinetic energy. If we are trying to get rid of forward recoil (from piston bounce) by having the projectile leave the barrel right at the point where the piston has reached near zero velocity then matching piston weight to spring strength is critical. This along with having a low friction between barrel and projectile.

[furstheimer]

It seems to me, (I could be completely wrong on this) that if the pellet has not exited the barrel before the piston rebounds that there would be a brief "sucking effect" on the pellet which might have a destabilizing effect from shot-to-shot depending on the pellet's ability to seal in the bore consistently.

[jkingrph]

It seems to me, (I could be completely wrong on this) that if the pellet has not exited the barrel before the piston rebounds that there would be a brief "sucking effect" on the pellet which might have a destabilizing effect from shot-to-shot depending on the pellet's ability to seal in the bore consistently.

I am by no means competent to really answer this, my background in medical science. but I would think the distance the piston would have to rebound to create your sucking effect would have to be a distance  so the volume of the cylinder creating the sucking effect would have to be greater than the volume in the barrel behind the pellet. I would think it would certainly decrease the pressure behind the pellet, but is that enough to overcome the inertia of the moving pellet.   This whole concept sounds like a bucket of worms to me.

[MartyMcFly]

It seems to me, (I could be completely wrong on this) that if the pellet has not exited the barrel before the piston rebounds that there would be a brief "sucking effect" on the pellet which might have a destabilizing effect from shot-to-shot depending on the pellet's ability to seal in the bore consistently.

I am by no means competent to really answer this, my background in medical science. but I would think the distance the piston would have to rebound to create your sucking effect would have to be a distance  so the volume of the cylinder creating the sucking effect would have to be greater than the volume in the barrel behind the pellet. I would think it would certainly decrease the pressure behind the pellet, but is that enough to overcome the inertia of the moving pellet.   This whole concept sounds like a bucket of worms to me.

The pellet is typically too far down the barrel when the piston bounces for the change in pressure to have a material impact. There are remedies that custom tuners have devised to reduce piston bounce. I’ve heard that Hector for example has a dead blow hammer design that drives the piston forward just when it’s about to bounce. Personally, I think a more desirable approach is to tune the spring, piston and transfer port to ensure that the piston comes to “gentle” stop at the end of the compression chamber. That said, it’s not an easy thing to do without extensive testing while varying preload, piston mass and TP diameter.

-Marty

[lefteyeshot]

It seems to me, (I could be completely wrong on this) that if the pellet has not exited the barrel before the piston rebounds that there would be a brief "sucking effect" on the pellet which might have a destabilizing effect from shot-to-shot depending on the pellet's ability to seal in the bore consistently.

I am by no means competent to really answer this, my background in medical science. but I would think the distance the piston would have to rebound to create your sucking effect would have to be a distance  so the volume of the cylinder creating the sucking effect would have to be greater than the volume in the barrel behind the pellet. I would think it would certainly decrease the pressure behind the pellet, but is that enough to overcome the inertia of the moving pellet.   This whole concept sounds like a bucket of worms to me.

The pellet is typically too far down the barrel when the piston bounces for the change in pressure to have a material impact. There are remedies that custom tuners have devised to reduce piston bounce. I’ve heard that Hector for example has a dead blow hammer design that drives the piston forward just when it’s about to bounce. Personally, I think a more desirable approach is to tune the spring, piston and transfer port to ensure that the piston comes to “gentle” stop at the end of the compression chamber. That said, it’s not an easy thing to do without extensive testing while varying preload, piston mass and TP diameter.

-Marty

You think?

[MartyMcFly]

It seems to me, (I could be completely wrong on this) that if the pellet has not exited the barrel before the piston rebounds that there would be a brief "sucking effect" on the pellet which might have a destabilizing effect from shot-to-shot depending on the pellet's ability to seal in the bore consistently.

I am by no means competent to really answer this, my background in medical science. but I would think the distance the piston would have to rebound to create your sucking effect would have to be a distance  so the volume of the cylinder creating the sucking effect would have to be greater than the volume in the barrel behind the pellet. I would think it would certainly decrease the pressure behind the pellet, but is that enough to overcome the inertia of the moving pellet.   This whole concept sounds like a bucket of worms to me.

The pellet is typically too far down the barrel when the piston bounces for the change in pressure to have a material impact. There are remedies that custom tuners have devised to reduce piston bounce. I’ve heard that Hector for example has a dead blow hammer design that drives the piston forward just when it’s about to bounce. Personally, I think a more desirable approach is to tune the spring, piston and transfer port to ensure that the piston comes to “gentle” stop at the end of the compression chamber. That said, it’s not an easy thing to do without extensive testing while varying preload, piston mass and TP diameter.

-Marty

You think?

I do! It’s the holy grail of tuning 😜

-Marty

[Yogi]

It seems to me, (I could be completely wrong on this) that if the pellet has not exited the barrel before the piston rebounds that there would be a brief "sucking effect" on the pellet which might have a destabilizing effect from shot-to-shot depending on the pellet's ability to seal in the bore consistently.

I am by no means competent to really answer this, my background in medical science. but I would think the distance the piston would have to rebound to create your sucking effect would have to be a distance  so the volume of the cylinder creating the sucking effect would have to be greater than the volume in the barrel behind the pellet. I would think it would certainly decrease the pressure behind the pellet, but is that enough to overcome the inertia of the moving pellet.   This whole concept sounds like a bucket of worms to me.

The pellet is typically too far down the barrel when the piston bounces for the change in pressure to have a material impact. There are remedies that custom tuners have devised to reduce piston bounce. I’ve heard that Hector for example has a dead blow hammer design that drives the piston forward just when it’s about to bounce. Personally, I think a more desirable approach is to tune the spring, piston and transfer port to ensure that the piston comes to “gentle” stop at the end of the compression chamber. That said, it’s not an easy thing to do without extensive testing while varying preload, piston mass and TP diameter.

-Marty

You think?

I do! It’s the holy grail of tuning 😜

-Marty

Absolutely,  balancing all those factors is what it is all about.  Tophat, spring guide, spring, is just the beginning.

-Y

[Mark 611]

I dragged out both of my Cometa 220 compacts in.177cal today both of them have barrels way shorter than the power plant, they are tune and sound like a PCP when fired, 1 still has the can on it the other I removed, both are factory barrel lengths 

[Yogi]

Sometimes I am just happy if the pellet leaves the barrel....

-Y

[UnderPressure]

It seems to me, (I could be completely wrong on this) that if the pellet has not exited the barrel before the piston rebounds that there would be a brief "sucking effect" on the pellet which might have a destabilizing effect from shot-to-shot depending on the pellet's ability to seal in the bore consistently.

I am by no means competent to really answer this, my background in medical science. but I would think the distance the piston would have to rebound to create your sucking effect would have to be a distance  so the volume of the cylinder creating the sucking effect would have to be greater than the volume in the barrel behind the pellet. I would think it would certainly decrease the pressure behind the pellet, but is that enough to overcome the inertia of the moving pellet.   This whole concept sounds like a bucket of worms to me.

The pellet is typically too far down the barrel when the piston bounces for the change in pressure to have a material impact. There are remedies that custom tuners have devised to reduce piston bounce. I’ve heard that Hector for example has a dead blow hammer design that drives the piston forward just when it’s about to bounce. Personally, I think a more desirable approach is to tune the spring, piston and transfer port to ensure that the piston comes to “gentle” stop at the end of the compression chamber. That said, it’s not an easy thing to do without extensive testing while varying preload, piston mass and TP diameter.

-Marty

You think?

I do! It’s the holy grail of tuning 😜

-Marty

Absolutely,  balancing all those factors is what it is all about.  Tophat, spring guide, spring, is just the beginning.

-Y

Can you name any examples of where this has occurred? (A rifle that has a piston which doesn't rebound (i.e. bounce off the cushion of air in the compression chamber) and instead makes a gentle stop at the end of the compression stroke before the projectile leaves the barrel.)

[mikeyb]

Pressure behind pellet is always positive during the shot cycle. It does drop and rise during piston bounce...because the pressure acting on the largely different areas causes the bounce! NEVER is pressure negative or trying to "vacuum" the pellet.

Piston bounce happens more often than most think. It is also NOT as big problem as most think. The 2 recoils occur before the pellet leaves the muzzle. Even during the ideal and rare "piston soft landing" condition, the 2nd recoil IS the piston landing! ... and the pellet is still in the bore.

SOME pellet energy is lost because of piston bounce but it is a SMALL percentage.

What the "soft landing" should provide is the MINIMUM 2nd recoil and the THEORETICAL best energy transfer to the pellet. If those marginal benefits can be obtained using SIMPLE COST EFFECTIVE tuning techniques then I'm in. Smoothest shot cycle, best accuracy, and then best pellet energy is what I aim for on every springer I own/tune.

However, if trying to gain ~1% more energy or ~1% less 2nd recoil I need to " redesign " a simple springer into some overly complex expensive failure prone mechanical nightmare then I'm out. So are most springer makers.

IMO springer manufacturers have known about springer physics for MANY years through empirical testing and more recently through detailed computational analysis and finite element simulation. Why "we" think we know MORE than they do without those deep resources is puzzling to me.

I'll sit back and lurk while the overthink wheels spin... eventually burn out.. and restart again in the near future. I smell overheating rubber already ;-)

For those NEW to springers, no offense intended. It is a deceptively simple system with a very complex behavior.

Folks with more springer experience should understand my comments.

For those who just like to dream of perfection, please keep dreaming (and reading and learning). Eventually one of those dreams may help make springers marginally better.

[UnderPressure]

Pressure behind pellet is always positive during the shot cycle. It does drop and rise during piston bounce...because the pressure acting on the largely different areas causes the bounce! NEVER is pressure negative or trying to "vacuum" the pellet.

Piston bounce happens more often than most think. It is also NOT as big problem as most think. The 2 recoils occur before the pellet leaves the muzzle. Even during the ideal and rare "piston soft landing" condition, the 2nd recoil IS the piston landing! ... and the pellet is still in the bore.

SOME pellet energy is lost because of piston bounce but it is a SMALL percentage.

What the "soft landing" should provide is the MINIMUM 2nd recoil and the THEORETICAL best energy transfer to the pellet. If those marginal benefits can be obtained using SIMPLE COST EFFECTIVE tuning techniques then I'm in. Smoothest shot cycle, best accuracy, and then best pellet energy is what I aim for on every springer I own/tune.

However, if trying to gain ~1% more energy or ~1% less 2nd recoil I need to " redesign " a simple springer into some overly complex expensive failure prone mechanical nightmare then I'm out. So are most springer makers.

IMO springer manufacturers have known about springer physics for MANY years through empirical testing and more recently through detailed computational analysis and finite element simulation. Why "we" think we know MORE than they do without those deep resources is puzzling to me.

I'll sit back and lurk while the overthink wheels spin... eventually burn out.. and restart again in the near future. I smell overheating rubber already ;-)

For those NEW to springers, no offense intended. It is a deceptively simple system with a very complex behavior.

Folks with more springer experience should understand my comments.

For those who just like to dream of perfection, please keep dreaming (and reading and learning). Eventually one of those dreams may help make springers marginally better.

Does this piston soft landing condition involve piston bounce? Or is it merely involve a piston pause (without a bounce) then the soft landing occurs?

[avator]

NT

[mikeyb]

Does this piston soft landing condition involve piston bounce? Or is it merely involve a piston pause (without a bounce) then the soft landing occurs?

There is no piston bounce in the "ideal soft landing case", but the second recoil is still present and the pellet still does not leave the barrel before the second recoil occurs. Basically the second recoil is the air being compressed and pushing back on the piston face. This is how a springer works. No second recoil=no air compression=no force to move pellet. You can minimize the second recoil by minimizing piston bounce but you can never eliminate the second recoil.

https://www.gatewaytoairguns.org/GTA/index.php?topic=207217.msg156435167#msg156435167

[avator]

I bet that Hollywood stunt man hopes the pellet don't leave his air bag too soon when he falls off the 20 story building.   

[Mark 611]

I guess in my case on my rifles, I do not even notice the second bounce from the pistons, but I have had some rifles where it was definitely noticeable, been a lot of interesting information posted, thanks guys! 

[mikeyb]

What we "feel" when shooting a springer is a firm nudge (force) into the shoulder as the piston is accelerated forward from rest. After about 8/1000 of a second the piston is coming to a very short stop on a high pressure slug of air lasting only about 1/1000 of a second. That abrupt deceleration can be felt as a forward surge force but can also be perceived as an abrupt HALT to the recoil force. Regardless of how it is perceived BOTH forces MUST OCCUR. It is the simple physics of force action/reaction in motion systems.

Because both forces are short duration and close together in "our" perceived time frame, shooting a well tuned springer will usually feel like a single* solid THUMP! It is important to note that the pellet is still traveling down the barrel when we feel this THUMP. About 1/1000 of a second AFTER the THUMP the pellet actually leaves the barrel.

In many cases the piston decelerates to a stop and reverses direction for a short distance. Theoretically this piston reversal can be eliminated to gain a little more pellet energy. That tuning effort is LARGE for a SMALL gain and will only work for a very narrow set of conditions. Change ANY of those conditions and a piston bounce will re-occur. It is NOT WORTH THE EFFORT to completely eliminate the "first" piston bounce.

AFTER the pellet leaves the barrel the piston returns from its short backwards trip onto an empty breech. This no-backpressure "collision" is not as large as the deceleration event because most of the system energy is already dissipated.  But the collision does vibrate the rifle and send shockwaves through the coil spring which can cause another smaller piston bounce. In total there can be several collisions/bounces after the shot with EACH smaller collision sending shockwaves up/down the coil spring length. THIS is what we perceive as a buzzy-twangy springer. The LATE piston bounces do NOT affect accuracy since the pellet has already exited the muzzle. The late bounces & buzzes DO affect spring life and can make the shooting experience unpleasant.

With MINIMAL effort, such as installing a properly fitted spring guide, the "late" bounces/collisions AND the associated coil spring shocks/vibrations can be reduced. IMO springer tuning to reduce these IS WORTH THE EFFORT!

-------------------- "Hacking The Benjamin Titan NP Break Barrel" thread ...

https://www.gatewaytoairguns.org/GTA/index.php?topic=176445.msg155995664#msg155995664
https://www.gatewaytoairguns.org/GTA/index.php?topic=176445.msg156102451#msg156102451
https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=176445.0;attach=348837;image

-------------------------- Travella's Published Papers...

https://www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns
https://www.researchgate.net/publication/277953420_Spring_Buzz_and_Failure_in_Spring_Piston_Airguns

--------------------------

Barrel length does play some part in all this. Depending on the swept volume and bore size the pellets in longer barrels (say 16"?) are usually STILL accelerating but MOST of the acceleration happens within the first 10" of barrel length. Does that mean we should ALL cut our springer barrels down to 10" or less?  I SAY NO!

Muzzle weights and barrel crops MAY improve a springers performance by changing barrel vibrations/harmonics. Changing the position and mass of a muzzle weight is non-destructive and reversible. There is no easy reversible way I know of to determine if a particular springer would benefit from a barrel length crop. It would be trial & error for each and every rifle. Crop an inch, recrown, test & record data for dozens of different pellets, then repeat. You will only find a sweet spot (if it exists) by cutting one time too short!

NONE of my springers is shooting bad enough to risk incremental barrel crops and expend the effort needed to document all the results. I'm happy to leave that effort for someone else.

edit: I found the slowmo video of a springer doing its dance. It was related to AOA testing the Diana ZR mount. You can watch the entire video normal speed but I suggest watching from the time linked at 0.25 speed. 
Presenter sounds WASTED if you listen to him talking at 0.25 speed. Good video overall ;-)


*A few years ago, before I really understood how springers work, I wanted to shoot a sub 10 fpe springer in my basement. I detuned an old B19 style springer by eliminating all of the spring preload. Normal spring preload in the rifle was about 2". That put it close to 6 fpe. It worked, barely, but the rifle had THE LONGEST shot cycle I've ever experienced. Trigger click, pause, THUMP, pause, pellet exit, pellet hits target, pause, ssshhh-thump#2 as the piston finally returned home from its one and only bounce! A really weak double-thumping springer. That's an extreme case but it is also the first time I realized piston bounce was a real thing. The B19 went back to full power and I found a perfect fit 5fpe youth springer for that project.

[HectorMedina]

I found the slowmo video of a springer doing its dance. It was related to AOA testing the Diana ZR mount. You can watch the entire video normal speed but I suggest watching from the time linked at 0.25 speed. 

Michael;

That video is flawed in many ways.

If you look closely to the times annotated in the video, you will note that the gun starts to move back at ms -408.8
Then the maximum excursion OF THE MOUNT is achieved at ms -398.1
Then the MOUNT bounces at ms 393.5 and then bounces again later at an undisclosed time.

Compare that shot cycle with the one in this video:



And you will see that the gun actually completes the shot cycle before the ZR mount can catch up with it.

;-)


Keep well and shoot straight!





HM