Are there any laws against shooting a paintball gun in your own back yard?

You need to be more specific. It depends on the area you live in mostly.

Of course.In addition to the state regulations about hunting and firearms possession, most communities have regulations about what weapons are allowed to be fired on property. Most do not allow...

No but common sense does need to be a factor. Obviously, an urban city like area would cause many problems. Though if you live in a wooded area, most counties do not care as long as you do not...

Not in general but if u get caught u probably get in trouble.

Unless you mean a simple potato gun, then not really. Designing a paintball gun from the ground up would take extensive machining and airsmith knowledge. Most people modify or customize existing...