After spending nearly an hour frustrated that I could not get a heart shot for the Target Practice quest, I think I've found the issue. I had been using elemental rifle ammo, and the first shot after switching to normal ammo was a heart shot. not sure if this is a bug or mechanic, but if intended I'd like to have that explained... because by this point in the game most people will only be using elemental ammo, as I was.
OMG. I've been trying for 4 hours or more to get a heartshot, had several before. But nothing worked. Tried Youtube, Google and so on. THIS. THIS worked. Changed to regular ammo and got a heartshot on the very next try. This needs to be fixed (e.g. a hint in the quest).