Can a heavy piston rob power?

[look no hands]

Oooooh....that's a big, big, sweeping question, Petey Boy!

Are you asking would fitting a heavier piston than optimal in a particular gun would rob power if converting? Maybe. Maybe not. As AB says, it'd certainly give a different feel to the cycle. If the spring wasn't changed, piston acceleration would be slower. But momentum would be increased. Recoil in the first forward stroke would fractionally increase, but piston bounce might decrease. Transfer port size and flow and pellet release pressure would also be big factors here. Depending on how optimal the piston weight was pre-fettle would dictate whether a lighter or heavier piston would be an improvement or otherwise. And it's often subjective and psychological anyway.

A heavier piston combined with more spring force and more preload could well result in greater power output but at a detriment to the firing cycle, usability and accuracy.

Depends on the gun.......I've often heard (but haven't experimented with this) that increasing the piston weight of rodless 25mm pistons (as fitted to Gamos and clones) might be helpful. But I quite like the quick feel of their cycles with their light pistons.

So much to consider here.....

I only ask as I'm sure I read JB's tuning of the Airsporter RB2 and he said the piston was too heavy, I was chronoing the Fenman last night and it is running at 10ftp with the drop in Diana gas ram, it was also doing the same with the spring kit I have, but that didn't have any preload slip washers fitted, I had a 5mm thick disk welded to the front of the Fenman piston so I could fit a Diana 35 seal, I first thought it might be too heavy but have just thought that it's been short stroked because of the longer seal and the added disk, so tat might be why it is lower power with spring fitted.