Reply To: found an air rifle under my shed. ..
Wish I could find a weihrauch under my shed. Like others have said take it apart, clean it(well). Relube properly then worry about pellet size. But if you have to know right away( as I likely would) go buy a tin each of cheap .177 and.22 and push a couple down the barrel. It will be obvious which one works. better yet if you have a friend who shoots air rifles bring it over to his place. I’m sure he wouldn’t mind letting you use a few pellets.