Lightening the ram/piston?

[Privateer]

An interesting post about this subject.
http://www.gatewaytoairguns.com/airguns/index.php?topic=2055.0

[Laser-Fist]

Gun shows up tomorrow morning, chrony a day later. I should have about 4hrs Sunday to log initial shots. I will have to break it in, and hopefully before July 4th I can get the O.E. piston out and modify it. I will try to reuse any swaggy China goo still hanging around the chamber and piston seal. Literally I would like to dissambe, rig the piston, drill it in my press, chamfer, regrease, assemble, shoot. Changing as little as possible, I have a feeling this gun is going to be apart a few times this summer. -Craig

[OleTomCat]

I would take it apart, clean, degrease, deburr and lube it first then start the testing...

After you get you preliminary data then start hacking at the piston...

[robtmc]

It's easy enough to gauge the effect using a chrony.

All well and good if velocity change is all you are looking for.  Did I mention velocity?

Robtmc has a good point to measure the psi at the transfer port hole. It would be great to measure BUT, too many dynamic things going on.

Maybe I did not spell out what I was interested in, very well.  Shorty may have hit on it here.

All mentions of markmanship problems with airguns seem to center around the relatively slow transit time from piston release to the pellet leaving the barrel. 

No idea at what sort of pressure in the chamber a pellet of any given weight starts to move, but high school physics is enough to know that once that starts, the force (pressure) that remains will continue to act on the pellet.  How long does that force act, or to the analogy to powder burners, does is continue through the event?

Benchrest shooter reloaders seeking ultimate precision spend many hours on something called cartridge neck tension, which affects bullet release, part of what is whirling around my aging brain as I think about airgun design and this light piston thing.

Point is, not velocity at all, but how fast, and at what rate, does the pressure from the piston act on the pellet.  This may not matter at all, or it may matter a lot with different pellet weights.  Does a lighter piston get the pellet out of the barrel faster and minimize the hold and vibration issues?

This is the idea of a pressure curve, and a correlation with pellet weights and accuracy.  Spank the pellet with perhaps a fast moving light piston, or give it a slower shove from a slower moving heavy piston?

Not trying to over-think things, but I was an engineer for decades, and was paid to do that.  Sorry if this is too much, I'll shut up.

[Paul68]

Fair enough.

It's well known that the time it takes a pellet to begin moving affects accuracy. In every spring gun there is a movement caused by the action of the piston release. This is why follow through on a shot it so important. Unlike a powder burner where the length of time between trigger pull and projectile movement is extremely fast, a spring gun is abysmally slow, relatively speaking. Although the point at which the pellet begins moving does play a role however, the greater problem is the movement caused by the release of the piston. The problem with spring guns is that you have to take into account all of the variables if you are going to achieve extreme accuracy.

For an example consider just lightening the piston. You will have reduced the mass of the moving assembly. Obviously, momentum will be reduced and the amount of force needed to overcome the inertia of that piston mass will also be reduced, so you get a faster cycle. Great, this should reduce recoil and improve "lock time". BUT, you also affect when peak pressure takes place and how high that pressure will be, and you have not changed the amount of force needed to break the pellet loose and begin its movement. So although you might have reduced the violence of the shot cycle, your peak pressure can be lower, and occur later in the shot cycle. You end up with a pellet that begins moving later, moves slower, and thus stays in the barrel longer. The longer it takes the pellets to travel the length of the barrel and finally exit, the more it is affected by the movement of the rifle and the harder it can become to maintain follow through. You then have to change the pellet to compensate, which generally means using a lighter pellet that requires less force to begin moving. So in effect, everything is dependent upon balance.

In effect, the best bet is to produce a gun that is performing as efficiently as possible, with as little movement as possible. If we were to assume accuracy were the only deciding factor in a spring gun, then obviously we would want something lightweight, low powered, and shooting light pellets. This is why the most accurate rifles are those with very low velocity and FPE numbers. If you really want short lock times, you could also reduce stroke, which of course would also further reduce energies and thus require yet more attention to balance.

There is a reason tuners and smiths don't pay much attention to trying to determine pressures and locktime, and instead focus on achieving a balance of power versus smoothness. Usually the best accuracy comes when a gun is producing it's peak power with a given pellet. From a magnum down to a low power target rifle, when you maximize efficiency and smoothness of the shot cycle, you improve accuracy. You get less movement to affect pellet trajectory, less energy lost to friction and unwanted movement and thus faster pellet launch, a pellet that leaves the barrel sooner, and typically a gun that is easier to shoot and more accurate. This is one reason why chrony testing is important. When you find the peak energies produced by the rifle, not velocity, and compare them to the energies produced by your alterations, you learn whether efficency has improved or been negatively impacted.

Guns are designed around a desired set of parameters and performance goals. Trying to take a gun designed to produce high energies and work well within hunting applications as well as plinking and turn it into a target rifle is a only going to meet with limited results. The best bet is to work towards achieving most balanced and efficient platform possible. Just lightening a piston is only a small part of the equation, and really just a step in the wrong direction with a given rifle imo because you are moving away from that rifles design parameters and trying to make a square peg fit a round hole. The best you can do is maximize a given platforms' potential. If you are after sheer accuracy then, better to start with a platform designed around producing accuracy.
 

[shorty]

I love busting paul68's butt. Your great man. I bet if we were neighbors, competition would never stop and many beers would be drunk in our garages or shops.

I say lighten the "dang" piston. I have proof it works. I have a chrony. I have chrony numbers. I ran a string of chrony tests on some new pellets I received yesterday that never have been shot out of my trail.

Impressively I must say. Baracuda hunter extreme 19.09 grain averaged 690fps giving 20.18 fpe. All the posts I have read about the trail np, I have never seen fpe numbers over 18 fpe.

This trail np or b18/19 platform is so much fun to work with.

Accuracy ? I am now convinced that the accuracy from this gun does not come from its engine. It comes from the barrel assembly, lock up, and barrel I/D. The barrel assembly or barrel itself is the accuracy problem.

for example: I just received FTT's 5.53 ,5.54,JSB jumbo's, crow mag's, all the good shooting trail pellets. I figured well, I'll tighten the barrel pivot screw along with the rest of the screws, give her a few drops of PFPE oil down the transfer port hole and all should be awesome. Well Chrony testing showed great numbers, but accuracy was *(&^. It took atleast 100 pellets to calm down then a thorough barrel cleaning to get back the accuracy after another 100 pellets. It was that "dang" barrel pivot screw that messed up everything. Stupid lock up system.

[aidave]

For an example consider just lightening the piston. You will have reduced the mass of the moving assembly. Obviously, momentum will be reduced and the amount of force needed to overcome the inertia of that piston mass will also be reduced, so you get a faster cycle. Great, this should reduce recoil and improve "lock time". BUT, you also affect when peak pressure takes place and how high that pressure will be, and you have not changed the amount of force needed to break the pellet loose and begin its movement. So although you might have reduced the violence of the shot cycle, your peak pressure can be lower, and occur later in the shot cycle. You end up with a pellet that begins moving later, moves slower, and thus stays in the barrel longer.

So to take this in another direction, you're saying if we increase the cycle time (heavier piston) we'd develop higher pressure?  Sorry, but the way I see it -- if the piston is lighter, it moves faster, develops pressure sooner (not later).

Just imagine if the weight were 5# the gas spring may push it, but it will move very slow and more pressure will 'escape' (or be lost) around the pellet

[Paul68]

Okay, think of it this way.

You have a 2 lbs hammer. You drop it on a nail from 4 feet. It smacks the nail in 1/4"

Now drop a 4lbs hammer on a nail from 4 feet. What do you think will happen and why?

[Paul68]

I love busting paul68's butt. Your great man. I bet if we were neighbors, competition would never stop and many beers would be drunk in our garages or shops.

I say lighten the "dang" piston. I have proof it works. I have a chrony. I have chrony numbers. I ran a string of chrony tests on some new pellets I received yesterday that never have been shot out of my trail.

Impressively I must say. Baracuda hunter extreme 19.09 grain averaged 690fps giving 20.18 fpe. All the posts I have read about the trail np, I have never seen fpe numbers over 18 fpe.

This trail np or b18/19 platform is so much fun to work with.

Accuracy ? I am now convinced that the accuracy from this gun does not come from its engine. It comes from the barrel assembly, lock up, and barrel I/D. The barrel assembly or barrel itself is the accuracy problem.

for example: I just received FTT's 5.53 ,5.54,JSB jumbo's, crow mag's, all the good shooting trail pellets. I figured well, I'll tighten the barrel pivot screw along with the rest of the screws, give her a few drops of PFPE oil down the transfer port hole and all should be awesome. Well Chrony testing showed great numbers, but accuracy was *(&^. It took atleast 100 pellets to calm down then a thorough barrel cleaning to get back the accuracy after another 100 pellets. It was that "dang" barrel pivot screw that messed up everything. Stupid lock up system.

That's fine and it's what I enjoy about this forum. I'm far from knowing everything and been wrong before, and learned because of it.

However, I have to say your results run contrary to what is usually found by lightening a piston, which makes me wonder what else is at play if your numbers are right and went up.

For instance, you mention introducing oil into the compression chamber. That alone suggests heavy deisling, which can easily boost FPE numbers unnaturally.

[shorty]

PFPE oil. Look it up. It is purchased from Solvay. M60 is the trade name. Non hyrdrocarbon lubricant.  Zero deizeling.Its a type of flouro polymer lubricant. Sounds like your calling BS on me.

What's it gonna take... I' ll prove it anyway you way you want. I got no reason to lie or B/S. Say it and I'll do it. No prob moustache. Now that was funny.

The results are contrary to springs. Maybe not gas rams..... They are different.

[robtmc]

Good writeup Paul.  Thanks for taking the time.

What bothers me a bit about what may be the tuners' process sounds like a lot of trial and error with parts swapping rather than any sort of formal engineering approach to things. 

Doubt that is what happens in the labs at RWS and all.  Then again, they may have a known set of basics that they stay close to.  In other words, the guts of the guns really does not change that much over time other than materials improvements and such.

I have noted on GTA that the pros never offer much info.  It is their bread and butter, not surprising they do not share any knowledge I suppose. 

[Privateer]

Okay, think of it this way.

You have a 2 lbs hammer. You drop it on a nail from 4 feet. It smacks the nail in 1/4"

Now drop a 4lbs hammer on a nail from 4 feet. What do you think will happen and why?

I don't think that applies to AGs myself.
Your talking about gravity and the effects of.
AG's with springs or pistons use a linier device to launch the mass that compresses the given swept volume.
That compressed air then launches the pellet. How tight that pellet is in the breech, and it's weight, most probably affects how soon it moves?


I do see that the Pros do offer advice.
I'd no sooner have a new Guy that read a few books on modding a 350 Chevy engine rebuild my engine then I'd sent a Gun to a new guy doing tunes.
They (like the masters at doing 350 Chevy engines) say right up front it takes time and many things to learn by hands on to get to thier level.

[shorty]

Here's what a chopped piston looks like. It's whats in my trail np hardwood right now.

[Paul68]

Okay, think of it this way.

You have a 2 lbs hammer. You drop it on a nail from 4 feet. It smacks the nail in 1/4"

Now drop a 4lbs hammer on a nail from 4 feet. What do you think will happen and why?

I don't think that applies to AGs myself.
Your talking about gravity and the effects of.
AG's with springs or pistons use a linier device to launch the mass that compresses the given swept volume.
That compressed air then launches the pellet. How tight that pellet is in the breech, and it's weight, most probably affects how soon it moves?

Doh!

No, not about gravity at all. It's about mass, kinetic energy, and inertia. Drop a ping pong ball on your foot from four feet, it wont hurt at all. Drop a 5lb weight from four feet, you'll probably break a couple toes. So what is different? They both drop from the same height and have the same exact force working on them.

The difference is the mass of the object that is falling. More mass, heavier object, means more inertia. Inertia is the tendency of a body to resist changes to it current state ie resting or motion.

How this applies to airguns and piston weight is simple.

The heavier piston will be better able to compress a given volume of air once it has begun moving and will resist the opposing force of high pressure better than a lighter one. It builds speed more slowly, but due to its larger mass absorbs more kinetic energy and achieves greater inertia.

A lighter piston has less mass and will not build as much inertia, thus it will not be able to compress a given volume of air to as high a pressure unless a lot more energy is applied to it.

This is why rifles with heavier pistons tend to be more efficient with heavier pellets and rifles with lighter pistons work better with lighter pellets.

Put a heavy pellet in a rifle with a light piston and the piston will bounce sooner and energy will be lost because the piston cannot overcome the pressure enough to efficiently launch the pellet. 

Basically, force equals mass times acceleration. Lighten the piston and you lose mass and only gain very little acceleration, so you end up with a net loss in force. You'd have to increase the force (stronger spring for higher acceleration) in order to achieve the same amount of force as you would with a heavier piston.

[Privateer]

 

OK. I got you now.
Thanks for explaining it better.
My brain don't work to good at times now days.
I'm trying to learn so if I'm off? Just whack me up side the noggin when needed.
The Wife does.
 

[shorty]

force equals mass time acceleration Basic? Not that basic with numb skulls. There's also static and dynamic friction occurring with the piston along with an increasing pressure. Iam not smart enough or have the equipment to measure. Lets look at that basic  F=MxA formula.

Force = Mass X Acceleration
100 = 100 grams (piston weight) X 1 inch a second ( fake number but Ok ) for giggles, let's just say this is whats in our trail.

Now lets lighten up that piston. Don't forget that the acceleration will increase.

110 = 80 grams (piston weight) X 1.375 inch a sec ( fake number but Ok ).

Could this be what is really happening ? The math says so. But, unless it can be measured, it doesn't mean poop. Just like privateer said. I would rather have a hands on engine builder build my 350 than some dude that just read a book about how to do it.

Something to think about huh.

[OleTomCat]

force equals mass time acceleration Basic? Not that basic with numb skulls. There's also static and dynamic friction occurring with the piston along with an increasing pressure. Iam not smart enough or have the equipment to measure. Lets look at that basic  F=MxA formula.

Force = Mass X Acceleration
100 = 100 grams (piston weight) X 1 inch a second ( fake number but Ok ) for giggles, let's just say this is whats in our trail.

Now lets lighten up that piston. Don't forget that the acceleration will increase.

110 = 80 grams (piston weight) X 1.375 inch a sec ( fake number but Ok ).

Could this be what is really happening ? The math says so. But, unless it can be measured, it doesn't mean poop. Just like privateer said. I would rather have a hands on engine builder build my 350 than some dude that just read a book about how to do it.

Something to think about huh.

This is why someone a page or two ago said to get out the chrony and get some numbers, then chop away and retest the gun with the same ammo over the same chrony....

In God We Trust, all others bring data....

[shorty]

Unfortinately, I got my chrony after I did the mod. I am not sure if more fps with lighter piston was even the main subject. Kind of rolled into that. I think the more positive thing to reducing the piston weight was the reduced recoil.

For me, I can say my recoil feels less. But I also say that my car feels faster when I changed my air filter. I also increase the transfer port size from .125" to .14". Even though this did reduce fps with light pellets (maintained heavy pellet weight fps) I can surely feel a lighter recoil with the larger transfer port hole. I did have the chrony for this testing and did test out 8 different pellets ranging in different grains from 12 up to 26.2 grains.

[aidave]

This is why someone a page or two ago said to get out the chrony and get some numbers, then chop away and retest the gun with the same ammo over the same chrony....

It would be interesting to test several weights -- I'm sure there is a diminishing return with regards to the weight of a piston.  I don't buy the "it would be too light to push the air" -- I bet if the gas-spring only pushed it 1/2 thru the stroke; that the heavier piston would go significantly farther. 

Since the spring pushes it all the way; the spring is doing "most of the work" -- given the same size piston, a rifle that requires 20# to cock and a rifle that requires 30# to cock will get different results -- it's the spring pushing, not the weight of the piston...

I am not a physicist nor do I play one, but it seems that lighting it to "some degree" would help recoil w/o damaging velocity or energy -- but go beyond that; it could.

If someone has a chronometer,tools to cut, scale, and the desire to experiment, I'd send the money for a new piston & seal.    That way we could see what a 5% 10%, 20%, 25% etc weight reduction yields with regards to pellet out the barrel...  just arm chair quarterbacking, but I bet about 40-50% weight reduction wouldn't harm end results...  Especially seeing how the quality of mfr'ing is -- why reduce the weight (more cutting, etc) when they don't have to?  They probably wouldn't reduce the weight, even if there was an improvement, since it would require more time in the build process....

[shorty]

aidave, Now that's an idea there. I would definitely take you up on that but Iam sure my other mods would void the test. But, I already have a cut piston and I wouldn't mind sharing that piston with someone. As long as I get it back or if someone gives me a replacement.

I am good with that.

Since I already have numbers with the cut piston and someone trades me with a stock one, the numbers can then be compared.