Give this a try. Bob as been really good about handling issues.
Customer Service:
Bob Ailes
Customer Support Manager
703-288-3500 ext. 122 <-- Best way to contact FNH
roberta@fnhusa.com <-- Least reliable way to contact FNH
I think the threads are M5x.08.
I've had similar problems getting thru to their customer service people in the past but once I did they took care of me. However FN blows when it comes to ordering spare parts alot of the time. It's impossible to get some things, like M249 parts.