Recently I stripped an earlyish gun (127xxxx serial no, around 1992/3 production) and also a later one (165xxxx, around 2005 production). The stroke of the 30mm diameter piston in the earlier rifle measured around 63mm, while that of the later gun came in at around 84mm. The piston mass for both was a portly 396g, while the inertia weight inside weighs around 73g.