These are the targets in the Scuba mission.
These two guys will NOT take damage until a couple of seconds after they are alerted to your position. I tried from a save right out in the water about ten times and could never kill them until I went around behind the truck altering them to my position.
Come out of the water nice and quiet and sneak to get a shot on them without the sirens going off. They will not take damage.
Please view the video here: http://www.youtube.com/watch?v=dkd50jXMx5g
It's because you're using the underwater ammunition. Notice that it's zeroed to 30m. Reload a STANAG and you should be able to damage them.
In the video the weapon was still using the underwater magazine types you have to switch to the 30rnd STANAGs to kill them
To the two posters, I was able to move around to the other side of them and kill them with no problems. I will load the save and see if that makes a difference. Thank you for the info.