Yeah I know what that is well. I think it is part 09 firing pin on the we scar. I had the same problem and replace it with a better version. The part that fallen from the receiver is the first picture with the white circle on it.
Here is the link
http://www.ratech.com.tw/store/produ...id_product=298 (part 11)
HIGHLY recommend to replace both part 09 and part 10. Part 11 is a combo thing.
Part 09:
http://www.ratech.com.tw/store/produ...id_product=296
Part 10:
http://www.ratech.com.tw/store/produ...id_product=297