Joule creep!

[Airsoft-Ed]

Ever since I started playing airsoft, and got involved in the whole game and science behind the guns and all this and that, to a deep enough level to understand it all, I have noticed joule creep.

But it's only as of late that I've started to see it earn a name, and a gathering of people who actually seem to understand what it is.

I first noticed it when I had an ICS L85 AEG, it was fitted with a 6.01mm tightbore barrel, 509mm long and fired .20s at about 360fps.
I noticed that when I loaded heavier ammo, my fps wouldn't drop by as much as the conversion charts said it would.

If you look at the equation used to work out muzzle energy, which involves the figures pertaining to ammo weight and speed, you'll notice that there's no mention of the length of the barrel, the bore width, or even the amount of pressure applied to the shot. Because it's just an equation, it isn't designed to be used in relation to airsoft guns, but it can be applied to them. As a result there are myriad variables it doesn't take into account, and this means that the conversion charts can be wildly inaccurate.

The conversion charts are simply that. They convert the muzzle energy into different weight and fps amounts. It's like an on paper reference to using scales. Adding more of one thing here will balance out this thing here.

The reality of it isn't so clear cut. Back when I first noticed it, the conversion charts said that loading .25s ought to reduce my fps by around 40, but I was seeing it drop by about 20. At the time I didn't realise that this would mean my muzzle energy was increasing, but it effectively meant that I was firing hot by around 20fps once I'd loaded the .25s. I was just counting my lucky stars that I was able to get away with having higher fps than the charts said I ought to be.

However, now I've owned gas guns for a year, and I've seen it happen a lot more extremely, I've come to realise that the ignorance of not understanding it could lead to someone getting hurt someday, so my aim with this is to try and help bring attention to joule creep, and bring attention to... not how to counter it, but how to play with it safely.

First, I'll try to better explain what it is and give an example of it.

Joule creep is the name given to muzzle energy which increases when the gun is loaded with a heavier weight of ammo. The name, joule "creep" is given because the energy, measured in joules "creeps" up without it necessarily looking as though the muzzle energy has increased.
It is common knowledge that adding heavier ammo reduces your fps, and many make the assumption that this drop in fps is a counter balancing effect which results in the muzzle energy remaining the same. You load .2s and chrono at 350, you load that same gun with .25s, you chrono at 315, muzzle energy remains the same. Right? Well, yes, sometimes.

Joule creep is caused by guns sending more pressure down the barrel than the barrel can contain. Let's set up a hypothetical scenario:

Gun A has a barrel which is long enough to contain 100% of the pressure that the mechanism sends down it in order to power the shot. So its cubic, volumetric capacity is large enough for all the pressure being sent down it to fit inside it.
If you fire a .2g BB through gun A, all the pressure will be used on the .2, the .2 will leave the barrel and there'll be no excess air leaving the barrel once the BB's free of it.
Adding heavier BBs to Gun A will see the conversion charts ringing true.

Gun B has a barrel which can't contain all the pressure sent down it to fire the shot. It's only big enough to contain about 50% of what gets sent through it to fire the shot.
So when loaded with a .2 it'll chrono at 350fps, but when the .2 is free of the barrel, it'll be followed out of the muzzle by 50% of wasted air.
This is a gun that will see a lot of joule creep, because now when you add a heavier shot, due to it taking longer to accelerate out of the barrel, it is present in the barrel for longer, which means some of that "spare" 50% can be applied to the shot, and because more pressure is building up behind the shot, it is actually increasing the power of it.
A .25 might use 60% of the air, a .30 75% a .46 95%... These are all just made up numbers to demonstrate the point, but can you see how a gun could gain power, that's muzzle energy, measured in joules. Kinetic, impact energy, that physically increases through the use of heavier ammo, all because the gun sends more power than required down the barrel.

Joule creep is therefore significantly more prevalent in gas guns than AEGs. This is because AEGs use pistons and cylinders. Cylinders can only contain x amount of air, and they can be ported to reduce the amount of air they send down the barrel. Generally a factory AEG will be fitted with a barrel and cylinder combo that more or less match the barrel and cylinder's volumetric capacity, which will prevent joule creep from happening.

Gas guns on the other hand, send significantly more gas down the barrel than required, simply by nature of the mechanism. It isn't something that can be reduced without it impacting the other performance characteristics of the gun.

The longer and tighter a barrel is in gas gun, the worse the effects of joule creep will be. A longer barrel means there's more enclosed space for the gas to expand in, and the tighter the barrel means there's less space around the BB for the gas to escape.

So I said I'd give some real world examples.

A few months ago I set up my gas L85 to chrono at just under 370fps on .20s, because that's my regular site's limit. My L85 at the time was fitted with a 509mm long, by 6.01 wide tight bore barrel.
Once set I changed to .28s which are what I generally run through it, and decided to chrono it again, just to gauge how much the fps dropped by.
To my amazement, it didn't drop. I got more or less the same reading. In the ballpark of 370fps on .28s... I thought I might've got my ammo or my mags confused, so I double checked, but nope...

According to conversion charts, a gun that chronos at 370fps on a .2 ought to drop to 312fps when loaded with .28s... But as I touched on above, this is because the equation assumes the same amount of pressure is being used to fire every weight of shot, but in gas guns, the heavier the shot, the more power gets applied to it. So My L85 therefore sends so much more pressure than it needs to down the barrel, that my fps remained the same because all the excess was making up for the drop caused by the weight increase.

So if I'd gone to field this, I would have been firing hot by around 60fps, which is a joule increase of about .40.

Today I thought I'd run the same test through my gas SCAR, so I chrono'd it on .20s, an average of 5 shots gave me a mean of 331fps.
Loaded it up with .3s and did the same again, mean average of 290fps.

According to conversion charts, 331fps on .20s ought to drop to 279fps, so that shows an increase in power of about 11fps from adding heavier ammo. Pretty significantly heavier too. My SCAR barrel is tiny too, I think it's 10" and it's not even a tight bore.
That's a joule increase of only .08 or so, so it's pretty damn insignificant, but it proves the point that longer and tighter barrels, as per my L85 can take much greater advantage of it.

Imagine if someone had a gas sniper, they set it up to fire at 500fps on .2s, it was fitted with a very long, tight as tight can be tightbore, and then they switched to .46s to play with...

Ouch.

So to be safe with your gas guns, ensure you look up the conversion charts, find the site's fps limits before you go, and work out what the limit will be on the weight you intend to use. Then when it comes to chrono, use the weight you intend to use, and set your fps to the limit you worked out. Conversion charts might not work with regard to the fps drop from adding weight being correct, but they are still correct in that x fps = x joules.

So as long as you're setting your fps using the ammo weight you intend to use, you'll be safely within the limits.

If you've read and understood everything I've just written out, or if you already knew about it, then help me spread the word. There are sites that ban the use of gas rifles because they don't understand the science behind the muzzle energy, or simply can't be bothered to deal with the need for people to chrono on the weight they say they're chrono-ing on. There are sites that force people to chrono using .2g BBs. This is ignorance, and it's potentially unsafe.

Given any opportunity joule creep needs explaining in a way that people can understand and pass on, because the more people know, the less silly rules and regulations they'll be stopping us from using the guns that we love, for the hobby we enjoy.

[BrightCandle]

The challenge for the sites is that some of them really want to ensure that the weight you say really is the weight that is fired. Combine that with the fact that all sites around me use a 0.20g defined limit (328 fps or 350 fps for automatic guns) and you have the recipe for problems we see. An unscrupulous person trying to get more range and accuracy can use the sites own system of testing with 0.20g BBs against them. All they need to is fit their gas gun with a tight barrel and get that nice over volume effect and then turn up and let the site use its own ammo (which lots do), or use 0.20g BBs for the chrono and bingo they are in with a massively hot gun when they put 0.28g's in there.

 

From the players perspective taking account of the effect and setting your gun for the appropriate muzzle energy is important, but we there is a wider problem of ensuring guns are safe before a match starts and this more than just testing with 0.20g BBs even if that is the standard. Sites need to be pretty confident the gun is tested with the actual BBs that will be run for the game and I don't really see any other way to go about it. I don't think a site can ensure this safety, a player can always test with one BB and use another in game and utilise an amount of joule creep, because the alternative is to lock the BBs they intend to use in a box and have them reload from it and only it. I don't think many gas gunners are going to want to be searched just to check they aren't using different BBs either!

 

Having had my finger broken before by a BB (well above power presumably) I tend to not trust other players and have a preference for sites that do a decent chrono test. But none of them is catching the joule creep issue because they are either just trusting a player when he says its a 0.28g BB in there or alternatively using their own 0.20g BBs. I don't know what the fix is, presumably testing gas guns with 0.28g BBs from the site, but then if the owner is really intending to use 0.25g BBs and has it set to the limit for those he is going to get turned away. Its not easy to fix the situation so I can see why sites might just choose to ban these guns that have this effect.

 

I personally think they also ought to test for rate of fire as well. I saw a polarstar last Saturday that was putting out a frightening rate of fire and I was glad he was on my team because it was a complete laser beam of BBs. This is also a safety issue as multiple strikes in the same place also cause a lot more damage as well, and the intention of all these limits is to avoid penetration of the skin. You get hit with a polarstar firing at 50 rps all in the same place and it could dig a hole in your arm, both from joule creep and ridiculous rate of fire. That is every bit as dangerous as the over power from a gas gun.

 

I think the only thing stopping these things being real incidents is that most players dress up pretty well to defend against the strikes and we don't see many gas or polarstar guns on the fields and those that do take the safety seriously. I genuinely believe that most of us really don't want to have the potential of hurting others so we pay due care and attention to this after tweaking our guns. But I don't think its currently practical for a site to ensure safety with gas guns, testing with 0.20g BBs isn't sufficient and testing with 0.28g BBs isn't fair if the intention is to use less. So it remains mostly in the gas gun owners hands to ensure they aren't utilising this to get an unfair and dangerous advantage.

[Airsoft-Ed]

*quote*

The trust issues can be avoided by making everyone chrono on the heaviest weight there is, instead of the lightest.

 

Joule creep will be exemplified the most by the heaviest ammo, so by forcing everyone to use that for the chrono, means their muzzle energy will only go down if they have a gun that over volumes the barrel. If their gun doesn't over volume the barrel then it won't make a difference one way or the other, but it will ensure everyone's within the limit.

[Sitting Duck]

Yup agree with Gun B over volume can produce really weird results or joule creep

if the cylinder to barrel volume ratio is out......

The whole ratio and getting correct cylinder/barrel can be a nightmare to work out from scratch

and even some apps/programs that are supposed to help can be open to weird misunderstandings

as it is the port that dictates the volume of air generated and some "3/4" cylinders are different to other "3/4"

 

In reality to working out volume ratios there is not a great deal of difference in cylinder diameters and using a tbb barrel or std

 

BEFORE anybody jumps in a tbb increases fps etc.....

 

I never said that - I said a tbb hardly changes the maths in working out the ratio or air

the tbb however makes the gun perform more efficiently and increase in fps & tighter groupings

(but does not change the ratio by hardly anything - aprox 1.5% - which is FA compared to wrong cylinder port used for barrel)

 

But yeah seen first hand the odd effects of say using a 363 cylinder on a 225 barrel - way over volume & low fps on .2's

might need to run some tests on that with heavier bb's as it could be prone to joule creep like you said

 

I dunno what the answer is to the chrono - there are numerous ways to cheat or bend the chrono stats

(not saying what you said was cheating chrono at all)

but think the old set in stone chrono method is perhaps the more basic simple method that most peeps go with

but is perhaps not completely perfect or flawless - bit like ukara/vcra I suppose

but can't see the sites willing to change or adapt their chrono tests as they just want to run a quick check & get on with it

 

It can be tweaked/cheated by incorrect volume ratio but still look legit or lamer on .2's like you said

but shoot higher on heavier bb's - same as hop on/off gives different results

so yeah far from perfect but can't see sites wanting to change or deviate from the normal quick chrono

 

Chrono's can be open for question or tweaking/bending the results but a rough chrono is better than a site not using a chrono I guess

(even if that does mean some results are not true accurate reflections of muzzle velocity in game with different weight of ammo)

[sp00n]

The trust issues can be avoided by making everyone chrono on the heaviest weight there is, instead of the lightest.

 

Joule creep will be exemplified the most by the heaviest ammo, so by forcing everyone to use that for the chrono, means their muzzle energy will only go down if they have a gun that over volumes the barrel. If their gun doesn't over volume the barrel then it won't make a difference one way or the other, but it will ensure everyone's within the limit.

 

very interesting point.

 

but just for the sake of my lack of knowledge on this subject, you have your gun set for 0.28's and the site chrono's @ 0.3's (yeah i know its only a small difference), but would that not make your gun chrono hot?

 

as from reading your original post, you set your gun for 0.2's, and found when using 0.28's you had the same FPS .... forgive my asking (and lack of knowledge) but would it matter what weight ammo you chrono'ed at as it seems you get a very similar FPS no matter what weight of bb's are used.

 

unless you are saying that you should set your gun up for 0.3's (for the chrono), then use 0.28's (in the game) ... in which case that makes sense to me

 

or have i got the wrong end of the stick?

[BrightCandle]

If you intended to use 0.25g BBs and they chronoed gas guns with 0.30's then you potentially come out as having a hot gun due to joule creep despite the fact that actually you would be within all day due to your ammo choice. You aren't wrong, it would penalise people who chose to use ammo below the maximum weight the gun could actually push effectively, they would have to use the same weight as the site chronoed with or set it up such that it was within with the heavier weight and then take a hit on the lighter ammo and be well below the limit. Its not really a good solution either.

[Airsoft-Ed]

 

very interesting point.

 

but just for the sake of my lack of knowledge on this subject, you have your gun set for 0.28's and the site chrono's @ 0.3's (yeah i know its only a small difference), but would that not make your gun chrono hot?

 

as from reading your original post, you set your gun for 0.2's, and found when using 0.28's you had the same FPS .... forgive my asking (and lack of knowledge) but would it matter what weight ammo you chrono'ed at as it seems you get a very similar FPS no matter what weight of bb's are used.

 

unless you are saying that you should set your gun up for 0.3's (for the chrono), then use 0.28's (in the game) ... in which case that makes sense to me

 

or have i got the wrong end of the stick?

Hmm, I see what you mean.

 

But fps limits are set because of the joules they relate to, not just because the shots travel at certain speeds. Conversion charts exist to tell you what fps you should be doing, and therefore, need to aim for if your gun suffers badly from joule creep.

 

So, bearing in mind that fps limits are set for the joules, if you chrono on a heavier weight, to prevent joule creep being a factor, and you chrono joule-safe. Then that means you'll be joule safe on every weight below that, and because being joule safe means you're under the limit, you can't possibly end up over the limit by changing back to lighter ammo, because the joule energy is only ever going to go down from lightening the shot.

 

You might end up very close to the fps limits on the lighter weights, but it should be impossible to be over them.

 

And if you are over them, then you can just lower yourself below them.

Sites could chrono with the heaviest weight available to ensure a person is joule safe, and then just to be on the safe side, chrono them again on their chosen, lighter ammo, just to be sure they don't somehow go over the fps limit...

 

Bit of a long winded way of doing things, but when the entire point of chronoing is for safety, not dotting all the Is and crossing all the Ts seems like it kinda makes the whole process pointless.

 

As mentioned in other comments, there aren't a lot of people using guns that this really relates to, so having to chrono them a handful of times to be sure isn't going to hold the day up all that much, and often when setting an NPAS it'll take a couple of trips to the chrono anyway because screwing a valve in and out isn't an exact science and getting it right on the first try is pretty rare.

[JamesAirsofterAgent]

Anyone else seen the huge paragraphs and thought feck this?

[TacMaster]

Anyone else seen the huge paragraphs and thought feck this?

 

[BBrotherwood]

Slightly off topic but does anyone find certain chrono's really dont like GBB? I have an xcortech something or another and I have some problems getting it to read shots with the L85. The site I played at today used the same one and in 10 rounds only one registered.

[Sitting Duck]

Yeah I do get a bit of TOUT on mine from time to time

can be a real fussy f*cker to be lined up and true to find a sweet spot

(then she behaves very well it seems)

 

not got a gas blowback only ebb R85 or gas non blowback pistols

but can't say I've noticed any of them being ultra fussy

[Airsoft-Ed]

Slightly off topic but does anyone find certain chrono's really dont like GBB? I have an xcortech something or another and I have some problems getting it to read shots with the L85. The site I played at today used the same one and in 10 rounds only one registered.

Sometimes if you're too close then the chrono can be confused slightly because it picks up on the spits of gas that pass through it along with the BB.

 

If you can hold it slightly further back then you should have fewer issues. About 10" or so is the ideal distance.

[Airsoft-Ed]