Can a heavy piston rob power?

[TonyL]

Does AA put these things in an TXHC to raise or rob power, simple question

I'd say to raise power as they use the same spec spring for all calibres, it's so they can adjust the power easier, exactly the same idea that BSA use with washers in the Supersport/star.

So I guess the simple answer is that a heavy piston doesn't rob power unless using extremes

:

As Pete says.

But it's so easy to generalise, as said earlier. Some guns might benefit from more piston weight, others not. It depends on the design, dimensions, desired power and characteristics etc.

In the case of the TX HC item, this longer / heavier top hat increases weight, and therefore momentum and also spring preload. Many tinkerers remove this heavy steel item and substitute for a lighter acetal item, giving very slightly less initial recoil from the first compression stroke and a "quicker" feeling cycle but have to increase preload to restore power as this usually drops, thus quickening up the cycle even more, due to quicker piston acceleration (lighter piston / increased spring force). That extra preload will also come into play to combat potentially greater piston bounce displacement.