This is the end cap of the over-the-barrel plenum as used on the FX Dynamic MK2.

It is a popular upgrade for older FX Panthera MK1, Dynamic MK1, and King PCP air rifles because it does not clamp around the barrel. Shortly after the launch of the original Dynamic system, it became apparent that the barrel clamp could negatively affect accuracy and was a cause of point-of-impact (POI) shift.

FX addressed this issue with this simpler, yet robust solution.