I think you are worrying over small details. The English-speaking audience to a very large degree don't read German or French, and I think Hans wrote the German book anyway.
1. Adam is right. I wrote the German CTHULHU WAFFEN-HANDBUCH, so there's a great amount of continuitiy between these products -- although this doesn't mean all the stats are identical (and all the text is completely new; I didn't simply translate it). In the years since the German book's publication, I've interviewed dozens of shooters of all descriptions (target shooters, soldiers, police officers, SWAT officers, Federal agents, etc, from four continents and dozens of countries and several generations), read hundreds (!) of new books, and shot many thousands of rounds myself, all of which helped to make a better book.
2. While there are differences in stats, they aren't substantial. Most are explained in the text or are so minor you won't even notice them.
3. We already have several sets of different stats, between the different editions of the basic rules, works like FATAL EXPERIMENTS or the 1990s HANDBOOK, and the non-Chaosium material such as DELTA GREEN. In fact, one of my aims with these new books is to provide a coherent set of stats that takes all the previous material into account and streamlines or corrects them where necessary, but otherwise leaves them untouched. Despite some of the obvious problems of the basic rules, I've made sure that my books are fully compatible, and most of my new rules are completely optional.
4. Finally, the INVESTIGATOR WEAPONS books are not, actually, all that much about game stats, but rather about use and users. Yes, I list the stats, cause you need them for play, and yes, I have tinkered with some of them. However, most of the text deals with questions like: Where would I find this weapon? What can I do with it? What does it cost, both retail and on the black market? Do I need a permit and what happens if I don't have one? What can I do in the way of accessories or special ammunition? Which kind of character might own one of these? How do you actually load or unjam it (I describe this step by step, almost like a manual)? How much time does this require both actually and in combat rounds? Etc, etc. No CALL OF CTHULHU book has ever lavished much attention on these details, many of which being actually more important to an investigator in his actual, day-to-day life than knowing whether the thing will do enough damage to a Shoggoth.