Are you referring to the KFS2000? Because if you are., it is not a prototype. It’s based on the FN F2000 which is an actual assault rifle that has been around since 2001.
It is also not unknown to the Battlefield franchise as it featured in BF2, BF3, BF4,BC and BC2. They probably had to rename it in BF2042 due to licensing issues.
I have just unlocked the gun myself so I still have to experience how powerful it is. The fact that you keep getting wiped by it could be perception as it is a new gun being introduced after a long content drought. So naturally players will want to experience the new gun for themselves which results in an increased usage.
But it would not be uncommon for a new gun to be OP, we’ve seen it before and it usually takes some time to get a balance pass.