Gun output is not always a constant. I have several gas guns where a change in BB weight did not change the velocity any real amount. Case in point: KSC USP Compact. I chrono'd the thing with 0.20, 0.25 and 0.36g BBs and the velocity stayed at about 300 fps +/- about 5 fps no matter what weight was used. Another case in point: KJW M700. I chrono'd that thing with .25 and .36g BBs, also with no large difference in output, at least not what you'd expect to see if you used a base energy output from the first test shots. Both weights were good for about 475 fps at 5 degrees C with green gas.
I have not gone to this level for an AEG, especially not a stock Marui or anything like that. But I do know from years ago that running .20 in a Marui would have netted an expect 290 fps vs. 0.25 at around 265 fps, but nothing more detailed than that.
Age verifier Northern Alberta
Democracy is two wolves and a sheep discussing what's for dinner.
Freedom is the wolves limping away while the sheep reloads.
Never confuse freedom with democracy.