All weapon accuracy stats are bugged, every single one of them, because of a basic coding error in where they're incrementing the number of shots fired. The only way you'll ever get a shot counted as fired from the PTRS, for instance, is if you've got the muzzle literally sticking inside someone when you fire. So the math bombs completely to avoid a divide by zero.
A lot of the achievements have similar problems of being set to trigger in incorrect manners, sometimes because of other bugs and sometimes just because they weren't done right in the first place.