Originally Posted by Belaslav
Let me explain in more detail. If I (for example) modify a spring airsoft rifle to fire at ~700 FPS, as long as it doesn't exceed 5.7J, it won't legally be considered a firearm, but it may give a reason for confiscation/testing in case someone gets a bleeding wound, wouldn't it? He will probably get it back, but still you'd probably want to avoid situations such as this as much as possible.
You can get a bleeding wound from sub-500fps gun.
Seriously, you've been spoon fed an explanation by several people, stop posting about his now; it's not what the thread is about.