My thinking is that the spring in the gun is probably shooting over 366 FPS for import, or possibly over 400 FPS.
The spring is either to bring the gun sub-350, or sub-400. My guess is that the gun is currently shooting over 400FPS, and the spring is included to bring it sub-400. But I can't find anything on the TA website about it.
The only way to really know, is to either chrono it yourself, or to call TA - I would call TA.