score:9
I have come to the conclusion that this bullet originated from an M61 Vulcan cannon. Developed shortly after WW2, and in common use over the last few decades in Switzerland. Even today the Swiss Airforce fly the F/A-18 with the M61 Vulcan cannon mounted. It is hard to place a timestamp on when the bullet was fired, given the many years of use. Based on the rust and location it was found, I can only guess that it was from quite a few years ago.