Reductions of the large headspace losses of the piston in this pistol has been experimented with by folks for years, using various methods, including the easy "aluminum tape" method mentioned recently by Lon (bigbore)...
Unfortunately, only minimal velocity increases -- e.g., single digit fps -- have generally been reported. Here's one reason why.
A significant headspace volume -- of the order of 0.01in3
-- is represented by the HW40 piston O-ring gland...
Stuffing the piston so that it contacts the end of the tube at the completion of the stroke, traps this air and blocks it from timely entry of the transfer port when the gun is fired, resulting in a decrease in total useful charge and disappointingly small (if any) gains in MV.
Here's one solution -- going ahead and stuffing the piston, but leaving a diametric channel that connects the transfer port to the piston edge, so that otherwise trapped and wasted air has a way to enter the valve and provide useful propulsion.
A nice increase in MV (17%) and ME (36%) appears (shooting CPLs)...
Unstuffed: 372fps = 2.4fpe
Stuffed: 434fps = 3.3fpe