N/A Muzzle energy varies with pellet weight, WHY?

[chromvis]

Hi I am new here I posted it in general discussion, but here it seems more appropriate.
I want to show my energy measurement results on 3 rifles with 16 different types of pellets (all 0.177 caliber)
1. Gamo Swarm Magnum Pro Gen3i (30 J energy according to Gamo)
2. Gamo Swarm Maxxim 10x Gen2 (20 J energy)
3. 760 Pumpmaster (old)

I shoot 16 different types of pellets with weight span from 0.325 gram to 1.31 gram, 5 shots each. Pellet weight was checked on analytical balances. An average weight distribution was within 1% accuracy.
Pellet velocity was measured with CHRONY Beta 2 inch from muzzle. Average velocity distribution was also close to 1%

BUT, velocity variation with pellet weight puzzles me!
Below are my data in graphical form

As you can see velocity drops with increase of the mass of the pellet - this is expected. What is unexpected is that it drops at significantly defferent rate for different guns, and for extra heavy 1.3 g pellets velocities fro all 3 guns are virtually the same!!!. Old 760 Pumpmaster with 10 pumps shoot 1.3 g pellet with the same speed as brand new the most powerful break barel rifle (Gamo Magnum).

Energy graph also show that that old Pumpmaster shows same stable muzzle energy independent on the used pellet, while muzzle energy of both Gsmo Swarm Magnum and Gamo Swarm Maxxim drops with increase of the pellet weight from 35 Joules (for Magnum) and 22 J (for Maxxim) down to the level of this old Pumpmaster with just 7 Joule.

Can anyone explain why it happens?

List of used pellets attached.

View attachment List of pellets.docx

 

[nervoustrig]

The energy decline with increasing pellet weight you are seeing from the two Gamo rifles is a behavior common to springer (or gas ram) power plants. It is related to the smallish volume of high pressure air available to accelerate the pellet.

Pneumatic rifles will generally produce increasing energy with ever heavier pellets, provided the valve is knocked open hard enough to provide an adequate volume of air to accelerate the projectile.

 

[charlie99]

It is because of the way the formula for muzzle energy works. Muzzle Energy (in foot-pounds) = (Bullet Weight in grains * Velocity squared in feet per second) / 450,400. For example, if a 230-grain bullet is traveling at 850 feet per second, the muzzle energy would be (230 * 850^2) / 450,400. So what that mean is if a projectile is lighter or heavier it will have different muzzle energy if traveling at same speed. The reason for speed dropping differently from different guns may be due to many factors including how efficient the air travels from valve to barrel and probable more so by how tight or loose a pellet or slug is in the bore. Just because a gun is .177, 22, .25 does not mean that each barrel will be identical. This is why 2 exact guns in the same caliber may shoot different using the same ammo and velocity.

 

[bustachip]

It is because of the way the formula for muzzle energy works. Muzzle Energy (in foot-pounds) = (Bullet Weight in grains * Velocity squared in feet per second) / 450,400. For example, if a 230-grain bullet is traveling at 850 feet per second, the muzzle energy would be (230 * 850^2) / 450,400. So what that mean is if a projectile is lighter or heavier it will have different muzzle energy if traveling at same speed. The reason for speed dropping differently from different guns may be due to many factors including how efficient the air travels from valve to barrel and probable more so by how tight or loose a pellet or slug is in the bore. Just because a gun is .177, 22, .25 does not mean that each barrel will be identical. This is why 2 exact guns in the same caliber may shoot different using the same ammo and velocity.

I always thought the constant was 450240.

 

[charlie99]

I copied and pasted that part from google, so if google has it wrong then idk.

 

[Scotchmo]

A piston gun use the inertia of the piston to hold the compressed air forward momentarily before that piston is forced back (piston bounce) by the compressed air. Ideally, the projectile leaves the barrel before the piston reverses direction significantly, thereby maximizing use of the compressed air. A heavier projectile accelerates slower, taking too much time, allowing the piston to be pushed backward before the projectile can get up to speed, wasting some of the energy from the compressed air.

If you increase the piston mass significantly, the piston gun will do better with heavier projectiles, at the expense of performance with lighter projectiles.

Most factory .177 piston guns are optimized for light to medium pellets.

 

[chromvis]

Thank you everyone. Unfortunately, all this does not explain why Gamo rifles show this energy drop, while Crossman Pumpmaster does not have even a slight drop. All these rifles operate through expansion of limited gas volume.
I only can explain this effect if Gamo being the break barrel rifle has gas leak through the crack and the heavier the pellet the longer acceleration an the more gas volume wasted though the leak.

 

[nervoustrig]

All these rifles operate through expansion of limited gas volume.

...the gas rams to a greater degree than the pumper. If a PCP were added to the graph, it would exhibit increasing energy with increasing pellet weight.

 

[Mycapt65]

Thank you everyone. Unfortunately, all this does not explain why Gamo rifles show this energy drop, while Crossman Pumpmaster does not have even a slight drop. All these rifles operate through expansion of limited gas volume.
I only can explain this effect if Gamo being the break barrel rifle has gas leak through the crack and the heavier the pellet the longer acceleration an the more gas volume wasted though the leak.

You need to read the replies 2 and 6 again. It was explained perfectly.

 

[DualMagMike]

I always thought the constant was 450240.

I have also seen, 450436 & 450249. AAFTA uses the 450436.

 

[chromvis]

You need to read the replies 2 and 6 again. It was explained perfectly.

#6 reply make sense, I never thought it would be so dramatic.

 

[CharlieF]

Here is something else to consider. In a PCP or Pumper (to a certain extent), when the trigger is pulled the air in the plenum is released instantly and the full force of the compressed air pushes the pellet out of the barrel. This normally is a consistent process. In a springer, as the piston moves forward it compresses the air behind the pellet and the pellet begins traveling down the barrel before full compression is reached. The pellet could be pushed forward a few inches as the piston moves forward and thereby reduce the compression of the air. The amount the pellet moves forward would vary by pellet weight. Heavier pellets would not moves as far as lighter ones. This could have an impact along with all the other explanations presented here.

 

[G-Man]

Hate to say it, but an air rifle, whether gas ram or pcp, will have a set (constant) energy value. As the pellet weight increases, the velocity will decrease and the energy as it leaves the barrel should be roughly the same. If you're getting more energy with heavier pellets, then the system is changing. You're measuring the weight of the pellets in grams... get a scale that you can weigh in grains and redo your experiment.

 

[Mycapt65]

Hate to say it, but an air rifle, whether gas ram or pcp, will have a set (constant) energy value. As the pellet weight increases, the velocity will decrease and the energy as it leaves the barrel should be roughly the same. If you're getting more energy with heavier pellets, then the system is changing. You're measuring the weight of the pellets in grams... get a scale that you can weigh in grains and redo your experiment.

This is absolutely incorrect for piston guns and already been explained why earlier in the thread.

 

[Mycapt65]

Here is something else to consider. In a PCP or Pumper (to a certain extent), when the trigger is pulled the air in the plenum is released instantly and the full force of the compressed air pushes the pellet out of the barrel. This normally is a consistent process. In a springer, as the piston moves forward it compresses the air behind the pellet and the pellet begins traveling down the barrel before full compression is reached. The pellet could be pushed forward a few inches as the piston moves forward and thereby reduce the compression of the air. The amount the pellet moves forward would vary by pellet weight. Heavier pellets would not moves as far as lighter ones. This could have an impact along with all the other explanations presented here.

This is true. There's several things to consider. Piston guns have many variables. Typically past a certain pressure point as pellets require more pressure to break free (start pressure) and move forward energy decreases. In part because the piston is rebounded harder on an air cushion. An increase in start pressure is normally associated with increasing pellet weight but pellet fit has a lot to do with it as well. This is why some pellets make more power than others in the same weight class. A looser fitting heavy pellet can make more power than a tight fitting lighter. Its a delicate balance.

Piston guns are very fussy when it comes to efficiency. You'd think opening a transfer port would reduce air restriction and increase power. Sometimes it does, sometimes it doesn't. Typically it doesn't. It increases lost volume which usually reduces peak pressure which usually reduces power. There are so many variables the only way to know what works is to try it.

 

[karl_h]

I always thought the constant was 450240.

That number can be found to be different in many places, sometimes rounding differences, one time long ago it changed with a more accurated definition of gravity. The number is basically 7000 grams squared * acceleration of gravity in ft/second squared. It doesn't matter, anything close doesn't change anything unless looking out to many decimal places.

 

[dgeesaman]

Thank you everyone. Unfortunately, all this does not explain why Gamo rifles show this energy drop, while Crossman Pumpmaster does not have even a slight drop. All these rifles operate through expansion of limited gas volume.
I only can explain this effect if Gamo being the break barrel rifle has gas leak through the crack and the heavier the pellet the longer acceleration a the more gas volume wasted though the leak.

A Pumpmaster produces the same pressure behind the pellet until the pellet is gone. Heavier pellets just take longer.

A spring piston produces a spike of pressure. The pressure falls off as the pellet starts to move. It can only be tuned well for a specific pellet weight. The goal is to match the pellet weight to the pressure spike. Too light or too heavy of a pellet will reduce efficiency and power transfer. So I can see how a spring piston will not produce equal power across a range of pellet weights.

David

 

[chromvis]

A Pumpmaster produces the same pressure behind the pellet until the pellet is gone. Heavier pellets just take longer.

A spring piston produces a spike of pressure. The pressure falls off as the pellet starts to move. It can only be tuned well for a specific pellet weight. The goal is to match the pellet weight to the pressure spike. Too light or too heavy of a pellet will reduce efficiency and power transfer.

David

Yeah, It is sort of pressure distribution between connected vessels with variable outside forces. Does anyone know of any physics literature about it? It should not be too complicated physics (Freshman level probably). The more I go into it the more I want to buy PCP!

 

[dgeesaman]

Yeah, It is sort of pressure distribution between connected vessels with variable outside forces. Does anyone know of any physics literature about it? It should not be too complicated physics (Freshman level probably). The more I go into it the more I want to buy PCP!

The physics of compressible flow is anything but freshman level. It’s probably not unlike flow through rocket nozzles..

I haven’t read it myself but you could start with Cardews Airgun: Trigger to Target

 

[MDriskill]

I can't add to the excellent notes above - and this is a detour from the OP's question - but I would note that heavier pellets tend to have better ballistic coefficients, and thus retain velocity better.

In other words, energy at the muzzle is not the whole story. The heavier pellet that starts with less energy, often delivers more downrange than the lighter one.