TL;DR Always chrono with the weight you will use for the day, with the hop set - no more issues.
Might sound silly but do hpa weapons carry energy for longer in the bb? Mine chronos at 330 but they sound loud still when they hit an object and they do hurt apparently.
Do you play with the same weight of ammo as you chrono with? Did you read proffrinks post above, pretty much covers it?
Simple answer: No. Complicated answer: No.
Actual answer: any excess air discharging a BB is wasted, if you put in a heavier BB, as that BB will stay in the barrel longer, some of the excess air will be applied to that BB. Thus that BB will have more active forces applied to it than the lighter BB, thus it exits the barrel with more force. HPA guns have, relative to AEGs - unrestricted access to excess air.
It is well established that a heavier BB will hold it's energy for longer, in AEGs there is a general trend (
due to cylinder volume) to follow the various tables for predicting the muzzle velocity of various weights of BB when you chrono with any particular weight.
However due to the way in which the force is generated in a HPA gun [Read: Options] and the intrinsic fact that a heavier BB takes longer to leave the barrel as it needs more applied force to get moving the same does not occur.
The problem is not an issue of some kind of magical hot gun effect,
joule creep can only occur when the player uses a BB weight other than which the gun was chrono'd with. (or modifies some other aspect of the gun post chrono - but I would consider that cheating not joule cree: vs using a different weight being just ignorance.)
The problem is as proffrink basically said above, a heavier BB will remain in the barrel longer than a lighter BB - In HPA guns , relatively speaking (vs an AEG), the method of propellent is fairly infinite/exponential.
The easiest practical example is that an overvolumed HPA gun firing a high PSI can propel two
different BB weights at the
same muzzle velocity - but the heavier one has the most energy. If you chrono on the lighter one, and then user a heavier one to play, that's a hot gun.
Now don't get me wrong, for the most part a HPA gun can be set up to operate to a similar table of predictable velocities, but it's not necessary and it has been demonstrated that HPA guns operating in this manner are more effective than not.
All the faults with HPA lie at the chronograph - cheaters will always be cheaters and HPA makes no difference to that,
but to chronograph players with any weight other than what they will use - that their hop is set for, is the mistake.