Thread from the WWn wiki - originally developed by Martianshark
If you own a motorized water gun (such as a Tarantula, Thunderstorm, Lightning Storm, or Electrostorm), you can do some electrical modifications to change its performance. This means increasing the voltage to give it greater range and power.
 You Will Need
Generally, you will need the following:
Batteries with more voltage than the ones you're supposed to use (a nine volt or two is usually a good choice)
A battery box/connection for your batteries
Wire (be sure it's the right gauge for the batteries you're using)
A soldering gun (or just use electrical tape)
This can sometimes be done as an external modification, but it's usually much easier to work with if you open it up. So take your screwdrivers and open it up. (or don't)
Remove the batteries (if there are any) from the battery box. Take your new batteries. If there's more than one of them, connect them to each other so that it's one, more powerful battery. Be sure the positive pieces connect to the negative pieces.
Connect the batteries with wire to the gun. This can be done by cutting off the battery box and connecting the wires directly to the wires inside the gun. Or, you could keep the battery box and connect your battery's wires to the battery box, where the stock batteries would normally go. Whichever method you choose, keep track of the positives and negatives, and be sure they line up correctly. For a more permanent and durable connection, solder the wires on.
To prevent short circuiting, wrap any exposed wire in electrical tape. If your battery is naked or doesn't have a waterproof battery box, wrap it in electrical tape as well. To secure the battery to the gun, you should also tape the battery onto the gun.
Now pull the trigger if you haven't already. The motor should be higher-pitched from the higher voltage, and your gun should now be more powerful. If it doesn't work, you did something wrong.
Now that your gun is modified, the voltage may be higher than what the stock motor is capable of handling. Fire only in short bursts, and don't hold down the trigger for a long amount of time. Don't use it for too long either.
If you can find a motor (such as an airsoft motor) that's the same shape and size as the stock motor, but is designed for higher voltage, you can put it in your gun and you won't have to worry about overheating.
And obviously, don't do something really stupid like hooking up your gun to a household socket.