It's been an interesting work. I seem to be the first to really go after the history of the airgun post google books. Everything being in German or French doesn't help either.
Another barrier is that there is little in the way of historical foundations. There are only a few books on the subject with some being rare.
It's not like a regular history topic with lots of books from lots of different sources. With airguns, just about everything is hidden away. Without google books, nothing could have been accomplished.