A German made gun will have proof marks under the barrel. Take off the forearm, if you open the gun after that is will come apart and you can take pictures of the marks. There should be a number like 7.8 or 9.1 to indicate the caliber, so it's diameter at the lands and will not match the nominal (groove) caliber like 8 and 9.3.
I used to love being able to hit hard at 1000 yards. As I get older I find hitting a mini ram at 200 yards with the 22 oddly more satisfying.