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How water guns work

Written by Silence
Last updated on 2010-05-29

All water guns use a system to generate pressure and push water out. This article details the individual parts and variations that are used in both homemade and commercial water guns. While it does not cover instructions for building one, it does provide information about design.

Separate chamber water guns

Separate chamber is the most common format for water guns. It allows the user to carry extra water, and it can be used with various pressure systems.


The reservoir is the main water tank in separate chamber water guns. It consists of a tank, a cap for refilling, and an outlet to the pump area.


The pump is the sliding device that sucks water out of the reservoir and forces it into the pressure chambers. The pump itself uses a plunger on a rod to seal a tube. The tube has an inlet from the reservoir and an outlet to the pressure chambers.

Check valves

Water guns have to control the flow of water so that water can go from the reservoir to the pump and then from the pump to the pressure chamber.Check valves, or one-way valves, use springs to seal off the flow of water in one direction. They ensure that water cannot return from the pressure chambers to the pump or from the pump to the reservoir, thereby losing pressure.

Pressure release valve

On the other hand, there are situations in which water must be released in order to reduce pressure. For example, pressure release valves are used in commercial water guns to eliminate excessive and potentially dangerous pressure. These valves also use springs to seal an opening. Internal pressure acts against the springs, and if the pressure reaches a certain threshold, the valve will unseal briefly to allow water to reenter the reservoir. Pressure release valves can make audible squeaks or hisses.

Homemade water guns generally do not need pressure release valves. These water guns are built with heavy-duty materials and are much stronger. The only limit on pressure comes from the user’s strength!

Pressure chambers

The pressure chambers (PCs) store the pressure generated by the pump. Various systems can be used, and they differ in terms of pressure dropoff, capacity, availability, and cost.

Air pressure

Air pressure system water guns are the most common, offering fairly reliable and consistent performance. More importantly, they may be the easiest to procure. Air pressure chambers are simply tanks that are full of air when empty. As water is pumped in, the displaced air compresses, generating pressure inversely proportional to the volume (from Boyle’s law). The tanks are inverted to let the denser water pool at the bottom, where the inlet/outlet is.

Variants on air pressure water guns often use plungers to separate the air and water. That way, the pressure chamber can be oriented horizontally for more efficient flow, as in Ben’s SuperCannon II [link here].

Some air pressure water guns also use pre-charger principles - in which extra air is pumped into the pressure chambers beforehand to reduce pressure dropoff. Use of plungers with pre-chargers is recommended in order to keep the extra air trapped inside after firing all the water out.

Elastic bladders

The constant pressure system (CPS), invented by Larami’s Bruce D’Andrade, uses the elasticity of rubber bladders to store pressure. Pumped water fills the bladder, which then exerts force and increases pressure. A bladder stretch in such a way that the pressure is nearly constant, so range dropoff as the bladder empties is nearly unnoticeable. Other advantages include that the bladder may be oriented in any manner.

Although bladders can be made in any shape, the most convenient, spherical and cylindrical, have been patented by Larami. A third shape, the diaphragm, uses a flat sheet of rubber clamped tightly to a solid surface that contains the inlet/outlet.


Spring pressure systems use metal springs or, theoretically, stretchable cords to exert force on a piston in the pressure chamber. These springs exert force that is proportional to how far the piston has moves (as per Hooke’s law), that is, they do not provide constant pressure.

Among commercial water guns, spring pressure is now found in the Water Warriors Splat Blaster, Water Warriors Steady Stream, and Super Soaker Quick Blast. Spring systems have likely been ignored due to their lack of power per unit weight, compared to air pressure systems.

Trigger valve/Nozzle

The trigger valve is a device that opens and closes the path of the water out of the pressure chambers. Ball valves are the most accepted because they can have full-sized bores that barely restrict flow. Ball valves use a large ball with an opening that can rotate to seal and unseal the path of flow. Most other valves, like hose handles, implement pull valves of the type used in modern commercial water guns. These valves have small plungers that are pulled out of or off of an opening to let water through.

When the trigger valve is opened, water flows out through a nozzle. The nozzle is a smooth aperture that refines the flow of the water and reduced turbulence in order to produce a more aerodynamic stream.

Pressurized reservoir water guns

Pressurized reservoir system water guns offer a slight variation on separate chamber water guns. Instead of using a pressure chamber separate from the reservoir, a PR water only has one tank, which serves as both pressure chamber and reservoir. The tank is filled with water, and a cap with no hole is used to preserve the seal. The inlet check valve for the pump sucks air directly from the atmosphere. Thus, pumping forces air directly into the pressure chamber.


  • Because only one tank is present, PR water guns are less complex.
  • For the same reason, PR water guns are smaller.


  • "Dead space" between the end of the pump plunger and the outflow check valve can exist. If lots of dead space is present, the air compressed in the pump will simply sit there and less pressure will be generated.
  • If the cap is opened, all generated pressure will be lost. Especially with a partly-empty tank, the user will have to pump a lot in order to generate pressure and fill up the empty space with air.

In the end, the PR system is only truly suitable for pistols and small sidearms. Because air is easily compressible, unlike water, PR is extremely inefficient in larger water guns because the pumping fluid will compress and fill up very little space.

Water cannons

Water cannons are much more powerful than PR water guns, but they use the same principle. A single chamber is filled with water, and then an air pump is used to pressurized the tank. Water cannons have large bores, large valves, and optimized nozzles.

Water cannons are usually used in a different manner than are PR water guns. While PR soakers are often lighter weapons, a cannon is designed to be pressurized only before the enemy is engaged. Water cannons are optimized to get maximum range and output in a single, solid shot. SuperCannon II, for instance, set the record for ranges of portable water guns.

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