I myself am in the process of getting the CFO to approve my project to build an AR15 receiver(Yes I have my restricted PAL)... It's actually rather simple since I'm gonna have it CNCd on a friend's torcam mill
... I probably wouldn't try "scratch" building an airsoft gun since most parts can be purchased.
(Oh and don't bother asking for the CAM/CAD files, Your not gonna get them...)