the easiest way to make a pneumatic gun is to get the barrel you want and wrap it with paper on one end until it is about 12 to 15mm diameter(then a wrap with rubberized tape like duct tape for grip) and stuff it in the barrel of any kind of nerf gun(make sure it is tight and snug). With the spring ...

I am thinking of doing a project similar to this. I want to make an airsoft gun (purely pneumatic) around this size. I am thinking of a simpler maybe 2 cylindar design. An important part is is must be pumped from an attached pump. Do you know the easiest way to make a compact high pressure pump?