I am also looking for one for a UMP and G36...
Small HK pins fit GSG-16 just fine. If you happened to buy one large pin for the stock you will have to enlarge the hole to make it fit
So if I'm understanding correctly these be used to replace both of the takedown pins on the GSG16?
I've seen this mentioned before elsewhere but I don't see an option for size in the link, are these the small ones?