Not just that, but especially in gas guns, joule creep is caused by the fact a heavier BB takes longer to accelerate, therefore is in the barrel longer, and therefore continues to accelerate as the gas keeps driving it forward.
With a GBBR you are releasing significantly more volume of gas per shot than with an AEG. Although rare, joule creep does happen on AEG's as well.
That's why we tell everyone to chrono with the weight of ammo they will be using in game, and simply convert their measured FPS and BB weight to muzzle energy.
As opposed to the old system, where everyone chrono's with .20s and we just ASSUME the joule curve is linear.
To give you a real world example;
My WE PDW shot 380fps when chrono'd with .20s, that's 1.34j
But on .28s, it shot 345fps, which is actually 1.54j
We had a field limit of 1.48j at the time, so you see how this could be a major problem.
And as for the original poster's question;
I use a systema PTW, it is an AEG on steroids and uses proprietary 120rnd mags.
I only carry 9 of said mags, and only ever fill them 50-60 rounds each.
I use semi only, because it's inherently more accurate, conserves more ammo, and is less likely to give away your position through noise and a steady line of BBs
Those 9 mags usually last me two 'deathmatch' scrims, around 30-45min each
I still get plenty of kills
Unfortunately, not everyone has an accurate laser rifle, many people, especially new people, with AEG's DO tend to favor full auto.
But that's part of the game, and you need to adapt your strategy to counter that.
You don't get good by handicapping other players, you only get better handicapping yourself!