Joule Creep - A public science experiment

[TheGrover]

Hey all,

 

I have been meaning to put a quick survey together for the airsoft community to feed into, so that i can develop a better understanding of the science behind Joule Creep.

 

So, i finally put one together in the hopes of attracting a few inquisitive minds, you can see the form HERE

 

For those that dont know what Joule Creep is, it is caused by using heavy ammunition in airsoft guns. it affects all guns, but i believe that gas powered guns are the most susceptible due to the way that they operate. the theory behind the issue is that a heavier BB is accelerated more slowly down the barrel, which results in it spending more time in the barrel, recieving a greater impulse and therefore carries more energy than a lighter BB, even if it leaves the barrel slower than a lighter BB.

 

what this means is that a sniper that passes a chrono at under 500fps on a 0.2g BB might actually be performing well over the safe levels of energy once you load it with heavier ammunition. this survey/science experiment aims to understand just how much of a difference it can make, and why.

 

if you can, please take any gas powered guns that you have (it doesnt matter if its a pistol, rifle, SMG or sniper, as long as its Green gas or propane that pushes the BB) to a chrono, and do a little science. once you have some results to share, fill them in here, completing the entire form once per gun.

 

the results will always be freely available to view and use, and once i have a few results in the spreadsheet i will aim to draw some conclusions here.

 

please spread the word, send the link to anybody you know who uses a gas gun and can spare a minute extra at the chrono station at their next skirmish

 

Thanks for your participation

[Sacarathe]

Is this the sort of information you're looking for?:

 

[TheGrover]

Actually, I don't think that table accounts for joule creep. From my own testing, going from 0.20s to 0.25s in my G5 increased the muzzle energy by around 8%, so the energy increased without altering the gun except changing the ammo.

[Sacarathe]

Actually, I don't think that table accounts for joule creep.

 

If not, then what is it displaying?

[TheGrover]

It's showing the energy of a bb fired at each velocity, so you can compare the different bbs, and figure out how a site limit translates when you're not using 0.20s.

 

Joule creep however is a phenomenon caused by using heavier ammo whereby the muzzle energy of a gun is greater than you would expect based on this table. My theory is that the mugger mass of the ammo means that it spends more time in the barrel as it accelerates slower, this results in a greater impulse being imparted by the propellant due to the way that a gas gun lets far more gas into the barrel than would fill it at normal pressure.

 

As a result, you could chrono a gun on 0.20s and be right on the limit, then be stopped later in the game for a random chrono test using your heavier ammo and find that you're actually over the limit.

 

Personally, I think this is a big issue when it comes to things like gas snipers and DMRs, where you're talking about high energies alreads, and adding another 10% accidentally might just give the heavier bbs enough energy to go through someone's eye pro, bearing in mind that most eye pro is only technically rated for a 0.20g bb travelling at less than 350 fps. I believe that marshalls should chrono guns on whatever ammo the user will be using, so that they don't actually start shooting hot in game, since very few people seem (at least in my experience) to understand joule creep. I once had to explain it to a Marshall who thought that joule creep worked the other way than it did, and insisted I chronoed on 0.20s since they have the most energy

[Sacarathe]

Joule creep however is a phenomenon caused by using heavier ammo whereby the muzzle energy of a gun is greater than you would expect based on this table.

 

I find that hard to understand.

 

 

 

bearing in mind that most eye pro is only technically rated for a 0.20g bb travelling at less than 350 fps.

 

How can you know that?

Edited May 25, 2015 by Sacarathe

[TheGrover]

There is no cheating, try it yourself, load a gas gun mag with a few bbs of heavy and light weight, you'll find that the heavier ammo has noticeably higher energy and lower velocity, since the bb is accelerated slower and spends more time with a torrent of high pressure gas pressing it along.

[Sacarathe]

I believe that marshalls should chrono guns on whatever ammo the user will be using, so that they don't actually start shooting hot in game, since very few people seem (at least in my experience) to understand joule creep. I once had to explain it to a Marshall who thought that joule creep worked the other way than it did, and insisted I chronoed on 0.20s since they have the most energy

Unless the person doing the test is aware that higher weights chrono lower velocities, they will put in a weight above .20g and then say it passed the test, when in fact it has not.

 

Why should a gun be tested at with higher weight BBs when the only things that matter are the FPS at .2g - or is there some trick to making heavier BBs cheat physics?

 

You can't cheat physics.

 

(I'm not arguing, but what you're saying does not line up with what others have said elsewhere, not saying you're wrong yet).

[TheGrover]

Okay I'm gonna try to explain it as best I can.

 

Due to Newtons laws, a more massive object (ie heavier ammo) accelerates more slowly than a less massive one when the same amount of force is exerted upon it.

 

The gas doesn't care how heavy the bb is. So it always applies about the same force on the bb wherever it is within the barrel.

 

Since the heavier ammo accelerates slower, it must spend more time in the barrel being accelerated by the same amount of force compared to lighter ammo.

 

This increased time results in a greater impulse (force multiplied by time) being imparted upon the ammo, which results in the projectile having more momentum, and more kinetic Energy.

 

The heavier ammo should always end up slower than the lighter ammo, because it accelerates slower, but the joule creep effect means it will not be as slow as the table would indicate.

 

My idea (which seems to have stalled) was to get some data together and determine how much difference the effect makes, and to try to figure out if the length or bore of the barrel affects it.

 

Edit: you're right, you can't cheat physics, but it's the relationship between the laws of physics which make things look as though they are

 

I did Physics at A level, and problems like these interested me for a while

[Sacarathe]

Since the heavier ammo accelerates slower, it must spend more time in the barrel being accelerated by the same amount of force compared to lighter ammo.

 

This increased time results in a greater impulse (force multiplied by time) being imparted upon the ammo, which results in the projectile having more momentum, and more kinetic Energy.

 

The heavier ammo should always end up slower than the lighter ammo, because it accelerates slower, but the joule creep effect means it will not be as slow as the table would indicate.

 

Ok, but how does that fool someone who expects a certain FPS from a certain BB weight to reflect a specified total joules.

 

What I am really asking is, if you have the FPS and the weight, how is what you're referring to relevant (i'm not saying it isn't)?

[TheGrover]

 

Ok, but how does that fool someone who expects a certain FPS from a certain BB weight to reflect a specified joule total.

Because the heavier bb will travel faster than their predicted FPS, and carry more energy. based on my own experiments, the difference is enough to make some gas guns shoot hot on heavy ammo, and within site limits on light ammo. Based on the idea that a site limit is in force to prevent guns being used that have muzzle energies powerful enough to break eye pro, since its the energy of a projectile, and it's momentum that determine if it will break through eye pro, not simply it's speed

[Sacarathe]

I apologise, it was my understanding joule creep was the way in which a person, irrespective of intent, may cheat chronograph testing to achieve game play with a gun which is above the joule limit of a skirmish site because that site measures their guns (in these tests) by FPS which does not reflect an accurate result if the person doing the testing is not familiar with the table above and does not realise that you need FPS AND an honest account of the weight of the BB fired to know for sure whether a gun is within the site's joule limits - thus the FPS (solely) is really irrelevant (as anyone can just always chrono with .30g and say they're .22g).

 

You are referring to something else (impulse) clearly.

 

 

Because the heavier bb will travel faster than their predicted FPS, and carry more energy. based on my own experiments, the difference is enough to make some gas guns shoot hot on heavy ammo, and within site limits on light ammo. Based on the idea that a site limit is in force to prevent guns being used that have muzzle energies powerful enough to break eye pro, since its the energy of a projectile, and it's momentum that determine if it will break through eye pro, not simply it's speed

 

Aye, but are you saying that this increase in speed will not be reflected in the FPS recorded (and checked against the weight) and thus make it impossible to chrono a gun? That doesn't make sense to me.

[TheGrover]

 

What I am really asking is, if you have the FPS and the weight, how is what you're referring to relevant (i'm not saying it isn't)?

it's relevant if a Marshall insists you chrono on 0.20s and then you load up the heavier ammo, you have changed the muzzle energy of the gun and increased it, so if that gun chronoed on or just below the site limit, heavy ammo will make the gun shoot hot based on its muzzle energy. That difference could be 20% or more if you doubled the weight of the ammo, I need the data to test that theory, which could mean that a gun that a marshal thought was safe was actually capable of penetrating someone's eye pro and doing some serious damage

[Sacarathe]

it's relevant if a Marshall insists you chrono on 0.20s and then you load up the heavier ammo, you have changed the muzzle energy of the gun and increased it, so if that gun chronoed on or just below the site limit, heavy ammo will make the gun shoot hot based on its muzzle energy. That difference could be 20% or more if you doubled the weight of the ammo,

 

Yes.

 

I need the data to test that theory, which could mean that a gun that a marshal thought was safe was actually capable of penetrating someone's eye pro and doing some serious damage

And this is why I question you, why do you need data if its a physics problem?

 

which could mean that a gun that a marshal thought was safe was actually capable of penetrating someone's eye pro and doing some serious damage

 

There has to be a degree of trust in this sport, based on everything I have learned in the last 2 months, using eye protection rated for bbs at .2g at 1 joule is nothing short of insane.

[TheGrover]

 

You are referring to something else (impulse) clearly.

Don't you see, impulse and energy go hand in hand. They are indirectly proportional, so increasing the impulse (aka momentum) of a projectile by 10% increases its energy by 5% (the energy increase will always be half the scale of the momentum increase, as the former is derived from its velocity squared and the latter is derived from the velocity)

[TheGrover]

 

And this is why I question you, why do you need data if its a physics problem?

Losing an eye is not a physics problem

 

And I want the data to figure out just how much difference joule creep can make to muzzle energy, and see just how likely it is that guns can be made to shoot hot by changing ammo, and if the effect is very consistent or if properties of the gun (bore, length, type of barrel for example) make joule creep more or less significant

[Sacarathe]

Don't you see, impulse and energy go hand in hand. They are indirectly proportional, so increasing the impulse (aka momentum) of a projectile by 10% increases its energy by 5% (the energy increase will always be half the scale of the momentum increase, as the former is derived from its velocity squared and the latter is derived from the velocity)

 

You have in your recent replies elucidated information which was not clear from your earlier posts which explains a lot, however I still query whether you cannot do this without gathering data.

 

If as you say, they are proportional, you should be able to figure this out without gathering data?

 

Yes: I'm not a maths/physics wiz, but I just cannot see how gathering data can help with this.

 

Remember FPS, is the velocity after the bb EXITS the barrel... the force of the BB after it leaves the gun - at this point it really doesn't matter what the internals of the gun were.

 

You cant cheat this. (though yes, it may not be possible to predict the FPS of a .3g bb even if you have a gun's FPS result from using .2g bbs)

 

I would suggest rather than asking people for data you just buy a bunch of different length barrels - not because your experiment is not worth the time, but because so much more can vary between results than just the barrel.

 

You need to control variables and polling for data will not do this.

 

Also, your form doesn't ask whether the hop up was on.

[TheGrover]

 

Yes.

 

And this is why I question you, why do you need data if its a physics problem?

 

 

There has to be a degree of trust in this sport, based on everything I have learned in the last 2 months, using eye protection rated for bbs at .2g at 1 joule is nothing short of insane.

eye pro rated for 1j is almost enough, since a 0.2g 350fps shot has an energy of about 1.13j. A sniper shot at 0.2g and 500fps has over 2.2j of energy. Bear in mind that the British Standard (EN 166 F) only guarantees protection up to 0.86j. Taking a hit from a sniper you're gonna want to be pretty Danny sure that his gun isn't running hot. And sure, someone can cheat their way into getting a hot gun on site, but it's the people who do it accidentally and unknowingly who this public Experiment is aimed at benefiting

[TheGrover]

 

You have in your recent replies elucidated information which was not clear from your earlier posts which explains a lot, however I still query whether you cannot do this without gathering data.

 

If as you say, they are proportional, you should be able to figure this out without gathering data?

 

Yes: I'm not a maths/physics wiz, but I just cannot see how gathering data can help with this. Remember FPS, is the velocity after the bb EXITS the barrel...

 

You cant cheat this. (though yes, it may not be possible to predict the FPS of a .3g bb even if you have a gun's FPS result from using .2g bbs)

There are two ways to figure out how joule creep affects muzzle energy. You are right. It is technically possible to do it with some calculations and a detailed understanding of hydraulics, gas physics and mechanics, but I don't have that understanding.

 

The other way is to get a load of guns and a load of ammo and test Them, and since I can't afford dozens of guns, I thought the people here could lend a hand.

 

Kind of reminds me of the Aperture Science way of doing things: don't botger with the Complex maths and research, just get your hands dirty and figure it out like a real man

[Sitting Duck]

I think it was Russell that said about a kind of way to cheat chrono....

 

Its a way of over volume or too much air expelled which is normally wasted on aeg's or can produce strange curves in flight....

 

It doesn't seem to apply when I tested a serious over volume AEG - full cylinder way too much for short-medium barrel

 

How ever it seems to apply to gas guns dunno about hpa

 

350 on .20's

should be about 310 to 315 on 0.25's

However I with creep the heavier bb's can be higher than expected lower figures

Yet the std chrono lighter 0.20's still stay at 350 or less

 

Even though heavier 0.25 or 0.30' emerge at say 345 which is usually an AEG pushing 400fps on 0.20's

 

Heavier bb's will travel better and hurt on close impact too

(Reason why snipers @425 - 450 have a MED of 30mtrs)

 

It seems to happen on gas but couldn't see any creep on 2 aeg's that were well overvolumed when I tested 0.20/25/30's on them a couple of months ago)

 

It might be concentrated gasses behave differently to normal air under pressure, which might explain why gas guns can get joule creep and not or not so much as airy fairy aeg's

 

This is quite possible 101% pure duck bollox but that is the theory I'm running with atm.

[TheGrover]

I guess the effect is only noticeable when the over voluming is beyond what an aeg can produce, since it relies on having the same propellant force throughout the entire travel down the barrel, so I guess that that means all gas guns (perhaps not CO2 ones if the valve closes well before the bb leaves the barrel) and I guess most hpa guns as well.

[Sitting Duck]

I think you could seriously over volume on aeg's.....

 

take a full cylinder box like an AK47 with 455mm barrel

then shove on a mp5k 110mm barrel = mofo over volume

 

I'm gonna go with say green gas might behave react differently as it hits end of barrel and expands as it hits air

or some other crap like that.....

 

dunno - don't care to watch an open university course on expansion of gasses as it reacts to air at speed malarky

just didn't seem to work for me on aeg - but was only about 100 to 125 mm over volumed

[TheGrover]

Maybe a crazy overvolumed aeg would do it. But I mean that a gas gun overvolumes in a whole different league, an AEG will never be able to over volume as much as a gas gun because of the high pressure of the green gas.

[Airsoft-Ed]

Wish I'd seen this a lot earlier, I don't have the patience to read through it all now lol.

 

The shortest explanation I can come up with is:

 

Joule Creep is an observable phenomenon that occurs when you chrono a gun with a higher cylinder to barrel volume ratio on .2s to pass chrono, and then change to a heavier weight to play.

 

So to break that down, it's important to understand that we don't actually chrono a gun for the fps reading, but the joules associated with that fps reading.

 

The faster and heavier something is, the more joules.

 

So fps limits are set because they put a cap on the max joules any gun on site ought to be putting out.

 

So, with that in mind. Let's say you've got a gas rifle - long barrel, powered by expanding gas.

 

On a .2, the shot is so light that it gets spat out using barely any of the power supplied at all.

 

I'm just making these figures up to explain it, but for argument's sake, let's say a .2 is accelerated out of the gun so easily that the time it spends in the barrel only leaves it time to have about 40% of the propellant sent down the barrel applied to it.

 

So now you've got 60% of that gun's power, shot to shot, just being wasted as surplus energy. It just comes out of the barrel after the shot and evaporates into the atmosphere.

 

Now the gun is chrono safe, or so they say, so the user loads up .3s to play, because they offer better wind resistance, accuracy and range.

 

This is where joule creep is dangerous.

 

People think this gun is safe, because it passed chrono. But, it passed chrono on a .2.

Adding more weight means that the 60% surplus power will now start to apply more of itself to the shots because it takes more energy to get them moving, they accelerate slower so they spend longer in the barrel.

 

The gun is now shooting hot in joule terms, despite having an observably lower fps than before.

 

Hence "creep" because the fps reading - should you chrono again to observe the fps drop or something - will be lower than before. But it won't be lower by as much as the conversion charts state, which means the joules will have actually increased, because the fps hasn't dropped by enough to balance it out.

 

The failing of the conversion charts is that they assume the same power is being used for each shot and each weight, when in reality, the weigh loaded literally dictates how much power gets applied to the shot, by the length of time it spends in the barrel.

 

The reason it's less apparent in AEGs is because they generally don't over volume unless the barrel is very short, and with short barrels, even heavier ammo isn't under enough concentrated pressure for long enough for it to gain a substantial amount of energy.

 

Also, a spring powered piston doesn't behave the same as expanding gas.

 

Air from a piston doesn't expand, so when it meets resistance from the BB being in the barrel, there's an air cushion between the piston head and the BB.

The BB being there actually acts sort of like a brake for the piston, limiting how fast it can move.

 

That's why AEGs sound different when they dry fire. Dry firing has the piston slapping the cylinder head with no resistance. Whereas when there's a BB in there, it hits home with a lot less force.

 

Where as gas doesn't give much of a shit what's around it, it's expanding whether you like it or not.

[proffrink]

This is why sites should really chrono on the weight you're going to use and then calculate the energy in Joules, rather than what some do and give you some .2s to put through the chrono.

 

I suppse that sets up a further need to trust people aren't saying they're using .2s when they're chronoing with .28s. I guess a set of scales would fix that but it's a lot of effort to go to.