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.
