Operating Pressure vs. Barrel Length

[ES13Raven]

I have asked this is some other forums, and different people give me different answers....

I'm wondering if there is a "sweetspot" relationship between your Input Pressure and your Barrel Length.......

With different Cockers, I have always used 14" barrels, and had my operating pressure around 250psi. This has always been a great combination for me, and I like the way the ball shoots with this setup. The ball just seems to "sail" and not dropoff.

Last time I played, I was using a stiffer valve spring and had my input pressure at 320psi. I did notice an increase in efficiency, but my Dark wasn't shooting like it has in the past. I was using the same paint, so I know it wasn't that.

Even though I was chrono'd at 295fps, it didn't seem like I was getting the same distance I was used to for so long.

So I'm wondering if running 320psi vs. 250psi with a 14" barrel, and having the air that hits the ball be more of a "pop" than a "push" is getting me less distance.

Here's my idea:

In my Cocker, the 14" barrel is more "efficient" at the 250psi operting pressure than the 320psi operating pressure.

At 250psi, the air that is hitting the ball is more Low Pressure / High Volume, more of a "push" and is accelerating the ball fully out of the barrel. At the point where the chrono reads the ball at 295fps, the ball has just peaked in velocity.

At 320psi, the air hitting the ball is a Higher Pressure / Lower Volume, more of a "pop" and accelerates the ball quicker. The ball reaches full acceleration while still inside the barrel, and actually starts decelerating before it leaves the barrel. At the point where the chrono reads the ball at 295fps, the ball is not at the peak of its velocity, but is already past it and is decelerating.

That could be why I felt I was getting less distance than normal at 295fps.....

Basically there is a certain barrel length that gets full ball acceleration for a certain psi.

Sound feasible?

Mind you, this is in a 14" barrel. Different barrel lengths will have different psi settings.

Ideas?

[ChucktheMAGician]

Sounds logical to me. I'm just waiting for the deep blue regulars to come and drop some science and equations on your theory to see how it turns up! Off topic but what reg(s) are you running on your cocker?

[ES13Raven]

I am running an AA Messiah

Originally posted by AGD
60 psi for the mag and the impulse
95 pis for angel
45 for matrix
110 for autococker

The next question we need to ask is, how far down the barrel does the ball have to go before the pressure gets to low to do anything useful? That answer is 8-10 inches. We know this from looking at the graphs that our gun dyno puts out. If your peak pressure is higher, say over 100 psi you can get away with a shorter barrel, if it's lower then you need a longer barrel.
AGD uses the precise contour of the power tube tip to release air in a controlled manner behind the ball to limit peak pressures to around 60-80 psi. So as far as our guns are concerned, the best efficiency would be had with an 8-10" effective length barrel.

So if 60-80 psi is the pressure coming out of the bolt on a mag, and 8-10" effective barrel length gets the best efficiency, the effective barrel length should be shorter for a cocker's 110psi right?

I assume the 110psi for a cocker is based on a stock cocker's operating psi? (350-450psi)

What about a cocker that has an operating pressure of 250psi? The pressure is lower, and dwell is longer so is the pressure coming out of the bolt closer to the Mag and Impulse?

A couple of other quotes I found interesting:

Originally posted by athomas
The barrel length is also a larger factor in lower pressure guns because the pressure is maintained for a longer period in order to allow slower acceleration to the desired fps setting.

A short barrel would be extremely inefficient on a low pressure gun. It takes a certain amout of torque to accelerate a ball to the desired speed. If the pressure is lower the rate of acceleration is lower. Therefore it takes the ball longer to reach the desired speed. In a short barrel, the distance is not there to allow that to happen. Therefore the pressure must be increased to allow a greater acceleration.

Originally posted by Paladin
Question: If shooting a paintball can generate a vaccuum in the barrel, what does that tell us about what is actually going on ? What it tells me is that the gun is not tuned very well because peak velocity happened long before the ball got near the muzzle. Then when the propelling gas runs out of push, the ball has to carry the rest of the way down the barrel on energy alone, sucking wind behind it and loosing/wasting energy. If the ball starts decelerating while it is still in the barrel it can't keep up with,or carry as far as a ball that doesn't start decelerating until it leaves the barrel.

Comments?

[Mr Orange]

Here is how I understand pressure. 250psi at 295 fps or 320psi at 295fps makes no difference at all. Once the ball leaves the barrel (or reaches porting) acceleration is ceased. so the very instant the ball leaves the BBL it is slowing down wether it started slowing in the barrel (say the last 2 or 3 inches after porting) or it is slowing once it leaves so long as it is actually measured at 295 fps the only things affecting the speed/distance now are atmospheric variances. Assuming they are the same then it shouldn't make a difference.

Hope this helps, If I didn't make it clear enough, post again and I will try to explain my self better.

PEACE
LT

[ES13Raven]

Well.... what I want to do is find the best operating pressure for my Cocker with a 14" barrel.

I want to have the operating pressure be efficient for the barrel length, and have the velocity peak near the end of the barrel like Glenn suggests.

[ES13Raven]

Originally posted by Mr Orange
Once the ball leaves the barrel (or reaches porting) acceleration is ceased.

Does it really cease at the porting, or is that the point where it takes a dramatic slowing?

So if I have a Freak barrel, and acceleration ceases at the step or porting, shouldn't I get the same velocity with the tip off?

Does that really happen though?

[AGD]

So why do you think input pressure has anything to do with the pressure behind the ball? You should look at the data thread carefully.

AGD

[ES13Raven]

Originally posted by AGD
So why do you think input pressure has anything to do with the pressure behind the ball? You should look at the data thread carefully.

So are you saying that all Autocockers have 110psi coming out of the bolt regardless of the operating pressure, valve dwell, and bolt type?

[Redkey]

without putting words in his mouth...

I think he is saying that alot can happen between the pressure gauge reporting 250 or 320 and the face of the bolt where the air blast hits the ball.

Every inch of hose, connector, elbow and orifice will change the rate of air flow and therefore change the pressure at the bolt face.

What are the main goals of optimizing the air pressure?

higher shot count? more consistant velocity? better accuracy? less noise? higher ROF?

Since your cocker is probably different from all the others out there I doubt you'll get the correct answer from this or any other forum. You'll have to determine the pressure that best fits your barrel length, barrel diameter, paint diameter, paint weight, valve type, valve spring rates, hammer spring rate, hammer weight, tank reg flow rate, temperature, humidity... etc. on your own. Once you've found this pressure through extensive testing... don't change anything on your marker or you might have to start all over.

[ES13Raven]

Originally posted by Redkey
What are the main goals of optimizing the air pressure?

higher shot count? more consistant velocity? better accuracy? less noise? higher ROF?

I know that the 320psi input pressure will get better efficiency, but that is not what I am after. I'd rather have a quieter marker at 250psi, than a louder one at 320psi and get 50 more shots out of it.

I know I won't get higher ROF by doing this, and the more consistant velocity/better accuracy is questionable.

I'm thinking that the 250psi operting pressure is allowing the paintball to reach peak velocity more towards the end of the 14" barrel.

Is Glenn wrong when he said this?:

Originally posted by Paladin
Question: If shooting a paintball can generate a vaccuum in the barrel, what does that tell us about what is actually going on ? What it tells me is that the gun is not tuned very well because peak velocity happened long before the ball got near the muzzle. Then when the propelling gas runs out of push, the ball has to carry the rest of the way down the barrel on energy alone, sucking wind behind it and loosing/wasting energy. If the ball starts decelerating while it is still in the barrel it can't keep up with,or carry as far as a ball that doesn't start decelerating until it leaves the barrel.

[athomas]

As mentioned earlier in this thread, a lot depends on the flow path between the input and the breach. The more efficient the flow path is, the more quickly the air is delivered. When the air is delivered in a more timely manner, the effect is that the pressure drop from input to breach is less. Therefore, the behind the ball pressure is more stable and controllable.

Most inefficient designs start with a high behind the ball peak pressure that quickly falls off due to flow restrictions. Many try to compensate using larger reservoirs to deliver a longer, lower pressure air blast. This is where the inefficiencies of low pressure guns comes from.

A lot of efficient guns are high pressure but have low behind the ball pressures. This is due to flow restrictions but is in part overcome by the higher input pressure which allows a consistent behind the ball pressure to be maintained longer.

The best design would use a fairly small volume of low pressure air, but would deliver it all at once. This scenario doesn't exist but many guns have gotten pretty efficient. Several of the cockers operate on very low pressure and do deliver fairly efficient operation as do many other brands as well.

Once you have the operational flow paths of the gun maximized, you select a barrel and then you tune your gun to match the operation that you wish to get. Ideally you should be able to get good efficiency and low noise. The efficiency will be directly related to the sustained behind the ball pressure and barrel length.

No matter what gun you have, you need a barrel that will maximize the acceleration profile you are providing. Otherwise, you will decrease the efficiency. The only way to know this is to try different combinations and settings. Find one that works for you.

If you are using a stepped, vented barrel, then the amount of acceleration past the porting is diminished. The stepped tip of the barrel only acts as a guide. The velocity of the ball at this time is probably only being maintained by the momentum of the air following it, or may even be slowing down. At any rate, the chrono reading you get is the true measure of consistency. No matter what anyone says. If the marker is shooting at 295fps, the ball will go as far as any other marker shooting at 295fps regardless of the acceleration or slowing in the barrel. At any given time once the ball has left the barrel, the frictional and gravitational forces are the same for all balls which are equal (ie; same brand, size and weight). Therefore they will slow down and fall at the same rate.

[Cristobal]

Good topic… but I haven’t seen any data or equations yet And I’m rather afraid we’re going to have to get into at least one or the other if we’re going to come to any conclusions. While I don’t have much data to throw at it, I can take a shot at understanding the mechanics of what’s happening. Note that this is going to build on and borrow from a bunch of other threads, including:

https://www.automags.org/forums/showt...threadid=18939
https://www.automags.org/forums/showt...threadid=20862
https://www.automags.org/forums/showt...threadid=33441
https://www.automags.org/forums/showt...threadid=55359
https://www.automags.org/forums/showt...threadid=64669


Now then:
A barrel is fundamentally a tube in which a paintball is accelerated by the force of a compressed air blast behind it. If we’re going to talk about sweet spots of barrel length vs. operating pressure we’re going to be talking about the characteristics of this acceleration. That means we’re going to have to talk about forces, pressures, and most importantly Energy.

First off, we’re going to need to get our concepts straight.

It was stated in a quote above that it takes a certain amount of torque to accelerate a ball. I would have to argue that this isn't the case. Torque is a measure of the impetuous applied to an object to get it to rotate. Torque will get a ball spinning (angular acceleration), but not propel it out of the barrel (linear acceleration).


What we're really talking about Energy. And energy means Work.

Quite simply, Work is the transfer of Energy. Specifically, we need to transfer energy stored in the compressed gas behind the valve into forward motion in the paintball as it travels down the barrel. This energy of motion is called Kinetic Energy (KE). The energy in the gas is a form of Potential Energy (PE).

For those of you who like equations:
KE = 1/2mv^2 where m is the mass of the object, and v is the velocity
PE(gas) = pV where p is the pressure of the gas, and V is the volume (note the difference between v and V)

If we know the mass of the paintball, and how fast we want to shoot it, we can find out how much Kinetic Energy it will need to have when it exits the barrel* All this energy has to come from somewhere: and it has to be taken from the potential energy of the gas. When the marker fires, it translates a certain amount of PE from the gas into KE in the ball. This energy transfer is the Work that the marker does on the ball, every time it fires.

The method by which this energy is transferred is the acceleration of the ball due to the pressure of the gas behind it. As the ball accelerates (and gains in KE), the gas expands in volume and decreases in pressure (loses PE). The ball accelerates because the pressure exerts a Force (F) on the ball. Fundamentally, Force is the change in momentum (the state of motion) of an object… otherwise expressed as its mass times its acceleration.

Again, for those of you who like equations:
Momentum = mv
Simply defined, acceleration is a change in velocity (more properly, the rate of change), so we can write force as:
F = ma

Now pressure is merely a Force acting over an area. For instance: psi = pounds (force) per square inch (area). So we can write:

p = F/A where A is the area. We can rewrite this to be:

F = pA = ma

(Note that for a paintball, the area that matters is the cross-sectional area, not half the surface area.)


Now we’re getting somewhere:

If we apply a pressure behind the ball as it travels (and accelerates) for some distance down the barrel we’ll transfer some amount of energy to the ball, which will now be in the form KE in the ball.

So the Work, the transfer of energy, is essentially the application of a Force (in this case a pressure force) over some distance (d).

W = F*d (note that the * is a multiplication sign)

The Work will be equal to the change in Kinetic Energy of the ball. If the ball didn’t have any movement energy to begin with, then this is merely equal to the final Kinetic Energy.

W = F*d = KE(final)

This has a direct application to paintball barrels: If we have a greater Force (i.e. greater pressure) we don’t need as much distance to accelerate the ball. If we have a lesser Force (less pressure behind the ball) we’ll need a longer barrel. It should also be noted that the pressure behind the ball is not necessarily equal to, or even proportional to the input pressure (see the gun dyno results AGD posted in the data thread.


In a perfectly efficient system, the PE of the gas behind the valve would directly translate into KE in the ball… but of course this isn’t the case. We can deduce this empirically by noting that some markers with the same input pressure get more shots per tank than others.

The more friction in the barrel, the more pressure force we have use to overcome it in order to maintain the same net Force on the ball… and so we end up doing more Work -- although the net Work will stay the same because the net force will be the same as long as we shoot at the same velocity.

W(net) = W(pressure) – W(friction) = change in KE


Furthermore, whatever pressure is left over when the ball leaves the barrel is essenitially potential energy that didn’t get used and will be equal to the pressure times the volume of the barrel. By the same token, whatever gas escapes or is otherwise routed so that it doesn’t go towards propelling the ball also decreases the efficiency of the cycle.

[ES13Raven]

Originally posted by Cristobal
It should also be noted that the pressure behind the ball is not necessarily equal to, or even proportional to the input pressure (see the gun dyno results AGD posted in the data thread.

Doesn't the Autococker's pressure behind the ball vary depending on what it's input pressure, spring setup, valve dwell and bolt type is?

[Cristobal]

Originally posted by ES13Raven
Doesn't the Autococker's pressure behind the ball vary depending on what it's input pressure, spring setup, valve dwell and bolt type is?

Sure... all I was trying to say is that just from looking at the limited data I can see in the dyno test results Tom posted, I'm guessing that the pressure behind the ball may not be directly proportional to the input pressure.

[ES13Raven]

Originally posted by Paladin
peak velocity happened long before the ball got near the muzzle. Then when the propelling gas runs out of push, the ball has to carry the rest of the way down the barrel on energy alone, sucking wind behind it and loosing/wasting energy. If the ball starts decelerating while it is still in the barrel it can't keep up with,or carry as far as a ball that doesn't start decelerating until it leaves the barrel.

So is this even possible?

If peak velocity occurs early in the barrel, and decelerates a lot while still in the barrel, but the chrono reads 295fps - will it still go as far as a ball where peak velocity occurs toward the end of the barrel and the chrono reads 295fps?

Can the paintball hold the momentum longer?

[bjjb99]

Originally posted by ES13Raven
If peak velocity occurs early in the barrel, and decelerates a lot while still in the barrel, but the chrono reads 295fps - will it still go as far as a ball where peak velocity occurs toward the end of the barrel and the chrono reads 295fps?

Assuming the same mass for both paintballs and nothing to affect the basic ballistic trajectory, yes.

Originally posted by ES13Raven
Can the paintball hold the momentum longer?

Not without reducing drag or increasing mass.

BJJB

[Natural Newbie]

IF it's possible, making a graph of effective barrel length vs. energy would tell you the "sweet spot" (where the graphs intersect). Correct?

[Cristobal]

Originally posted by Natural Newbie
IF it's possible, making a graph of effective barrel length vs. energy would tell you the "sweet spot" (where the graphs intersect). Correct?

Sure. What you could do would be to graph effective barrel length vs. the rate of energy release. If you dump your energy quickly you'll accelerate the ball quickly and need less length, dump it slower and you'll need more barrel length.

in a sense it would be length vs. peak pressure behind the ball (for a give type of pressure release -- dependent on the valve characteristics)


In a related matter, it would be a fun deep blue excercise to calculate the theoretical amount of Work necessary to accelerate a paintball to 300fps against the force of air resistance and friction in the barrel.

The Work would give us the energy transfer, and then by dividing by the time it took to transfer that energy (time paintball is in the effective length of the barrel) we would arrive at the the average Power delivered by the marker in shooting a paintball. (Power simply being the rate of energy transfer -- how much energy that can be transfered in a given time) It would be fun trivia to know the horsepower rating your paintmarker

[ES13Raven]

Originally posted by Cristobal
in a sense it would be length vs. peak pressure behind the ball (for a give type of pressure release -- dependent on the valve characteristics)

I guess that is what my original question needed to be....

Some markers have fixed pressures coming out of the bolt, and have a certain "sweetspot" barrel length, but the Cocker is different because the pressure can vary depending on setup.