RPT-31 Unlock progress jammed

So I have been working on RPT-31 since last season, however the "using BSV-M kill enemy over 75m" part has still not been achieved, I try to zero some enemies even over 100m in both single-player and official conquest but the progress is still 13/30, seems the kills just doesn't count into the progress, anyone has the similar question? I really want to use this gun, it's like the only locked gun in my inventory.