BB Weight is all designed for the FPS out-put of your gun,
The higher your FPS, the heavier BB you are going to need in order to maintain a stable and accurate flight Path ( Trajectory ).
This subject has come up so many times it´s not funny anymore, use the SEARCH and you will find !!!
To use both ends of the spectrum, Nobody is going to be using .12´s in a gun that is shooting 650 FPS !!! same goes in the other direction .88´s in a gun shoothing 200 FPS !!!
Your FPS+BB weight+Hop-up+BB quality+Shooter Skill is what is going to affect your accuracy !
A crappy gun, with substandard BB´s that are either too heavy or too light, in the hands of a Players who has no Idea what he/she is doing is nothing more than an Easy Target.
There are plenty of BB weight tables and charts on the Net, a good source for a starting point, but based on the gun, BB´s, Skill, they should only be used for reference.