[ThunderCactus]

A GBBR does the exact same thing an AEG does when it has a full cylinder on a 363mm barrel.
It's just undervoluming the crap out of it's barrel so when you use heavy ammo, the muzzle energy rises.
However, GBBRs do absolutely have an upper limit. They actually have a fixed amount of gas that gets fired into the barrel every shot, which lessens as the gun cools down.
The thing is, that upper limit is pretty big since they use a pretty big volume of gas to fire the BB.
They're really no different than AEG's. Different firing mechanism, different power source, exact same physics.

Interestingly enough though, on AEG's particularly there's a point where consecutively heavier ammo starts to lose muzzle energy, and it's due to the air pressure not being high enough. You can overvolume it, but without the necessary pressure there's a point where it just start to joule creep in reverse again.