this might be an issue with the game detecting whether you are underwater or not, because most underwater weapons have a range of about 40-50m after which the bullet slows down exponentially and then just sinks, which in game means your bullets just kind of disappear. Were you completely out of the water? I know when i played the mission the first shots i fired where after I got to a shed about 50m from the shore and i killed a guy at a 100-150m range.
edit: the guy below me probably has the answer, but I won't remove the note just in case.

Did you change to the STANAG rounds because the defualt rounds are only for underwater

This happens even if you are several hundred meters from nearest pool of water.
I did not switch rounds, actually i did not even know there is such option lol.

Yeah use the scroll wheel to switch from the 20 round dual purpose underwater magazines to the 30 round STANAG magazines which should kill them just fine

Thanks, gotta try to pay more attention to the scroll menu :)

Closing, as this seems to be a non-issue.