Ok, talking about TAGs: Why can't warships fire their beams more accurately at TAGged ships? I once had an idea to make a mission where the player has to TAG the enemy warships(which are outside 4,000 kilometres) so that friendly warships could have an extended range. It did not work.
If the beams didn't fire its because the weapon range in the table is too low. Even if a ship is tagged and you tell the turrets to fire on tagged ships, they have to be within the weapon range of the turret else they will not fire.
Try jacking up the range or you might be able to get it to work with the beam-fire sexp.
BTW, the SCP comment was aimed at aldo.

@GRG: Yeah, check the retail table. They have the Tag C written in but it's not player allowed.