Airsoft gun velocity

Factual Questions
1

I’m looking at an airsoft gun that has an advertised velocity of 400 fps with .12g BBs. The place I’m going to limits velocity to 385 fps. Figuring that energy is constant, if I use .20g BBs then the velocity when tested should be sqrt[(12/20) x 400^2] = 310 fps.

Any errors in this logic/calculation?

2

Heavier loads tend to be more efficient but you should be pretty close. I wouldn’t be surprised if the 20 grain maybe hit 330 fps.