What caliber gun was used in the Emsdetten school shooting?

Answer 1

News reports weren't specific as to the caliber.

This is the closest I was able to find:

"...The Münster police said they had found three seemingly historical firearms with the teenager's body. Two of the guns were so-called "percussion weapons." These are built according to antique constructions designed before 1870 and can be purchased without a permit. The third, a small-bore rifle, required a gun license."