The pre-charge system is used in the Aqua Masters Pre-Chargers, such as the Argon.
Pros:
Low number of pumps
You are not exactly pressurizing when you pump, but merely filling the firing chamber with water
Creates CPS like shot, without drop-off
Cons:
Pressure is not quite constant
Would be hard to construct by hand due to the way it was constructed
How it works:
1. Air is pumped into the back of the firing chamber, behind a divider, separating air from water.
2. As soon as enough pressure is achieved, water is pumped into the front of the chamber, while pushing the divider back, to pressurize the air further.
3. When the trigger is pulled, the trigger valve opens and the pressurized air forces the water out the nozzle, but leaving the air behind the divider, limiting mist shots.