It's for taking out people behind cover mainly.
It explodes 3m past the point that you aim it, so if someone's behind a pillar or something, you'd set the distance by aiming at the pillar (look at the meter distance on the display) and then look to the side of the pillar and fire, it should take out the guy behind it.
Youtube it for a better explanation/demo.
Ahhhhh I see!!
Will try aiming it in front of people and not at them then, it only takes +2 damage from a tank
That said the law only does 21 damage too.