How to Build a Shooting Net Gun
Here is a nice tutorial detailing how to build a shooting net. The gun releases a net that is designed to entrap a target. Its quite a complex project and may set you back a few pounds to gather the necessary parts to devise the gun and a lot of time to put it together.
The project itself is mainly divided into 4 main parts, building the pressure chamber, trigger valve, the launcher arms and finally the net and testing phase in between each. But listing these points down like this does not do justice to the hard work and thought that would have been required to build the gun as it is quite a complex project. You can find the detailed instructions on how to build a net gun on the Instructables site.
Its a powerful gun capable of launching a net that is a massive 90 square feet a distance of up to 25 feet.
Though it is advertised as a self-defence and security device in some places, that might in fact be quite impractical unless it is used by law enforcement or security because it requires time and planning to use it effectively. It might be more prudent to use it on hunting expeditions or the like.
Unless you are of a kind who likes to put together interesting projects, if you really wanted this gun, it might be worthwhile just to purchase one.