This is an untested product with unknown effects. My hypothesis is that it would increase the FPS. This would theoretically be achieved because the decreased volume being depleted faster allowing the nozzle to return rearwards sooner. It can also make your engine more air efficient while using shorter barrels, since the fixed volume portion of the engine will be smaller.
This is all theoretical, I haven't tested anything other than fitment.
THIS PIECE REPLACES THE O RING IN FRONT OF YOUR SPRING - feel free to try both with and without. The spring buffer o-ring is not required when using this volume restrictor - it may be used but is not required.