I found this bullet frozen in glacial ice in Switzerland. There is no visible stamp on it anymore and unfortunately did not have any precise measuring equipment with me. I can measure the size of my ice-axe to get some estimation of the caliber if that would help.
My thoughts are that it probably originates from an aircraft cannon or anti-air gun. Although there is a small ring on the bottom of the bullet, it looks too small to be a primer, so I assume this is just the bullet part, without the casing.