I just redid the basics of our old timer Hannibalito 3.
As written in the webpage, this is a veteran with 8 years of life in it.
The battle capable, but unarmored weight is 6.2 kg.

Weight you can't avoid is the CO2 bottle. Unless you want to try the still non succesfull car-compressor route.
A 20oz Paintball bottle with on/off valve is 1.5kg full. A 12oz is a lot lighter with 1.2 kg.

A FP capable 2/2 valve is 700 gram.

Ram. Depends on diameter and stroke, but unless you go full CNC custom build, you won't get lower than 1 kg.
Using a Hydraulic ram in suitable dimensions is a lot worse.

Low pressure with commercial parts is equaly annoying in size and weight. Customisation is almost mandatory.
Finding a usable pressure regulator is a pain in the posterior in its own.