I build all my guns with an automatic safty device.. whenever the rubbers arent loaded, the device automatically kicks into safe mode
But on a serious note.. if u have to have a false sense of security, oh i mean safty then the easiest thing to do is to drill a small hole about 3 or 4 mm diameter that goes from one side and out the other right behind the trigger and as high as u can drill it close to the trigger mech body.. then all u have to do is put a pin thru which will stop the trigger from moving backwards.. also u can drill a hole in the end of the pin and thread some dyneema or a crimped mono loop for ease of pulling the pin out.
Hope this helps!
Good Luck
Zane...
