I don't think I have a kneecapper shotgun... might have stashed it in my "special gun cabinet" though
In the end I got pissed off and took a fatman to it's face. That showed it, though the speed of its death messed up the spawning of the legendary fellas, they decided to pop up randomly later
That Mirelurk Queen... with two legendary Mirelurks... I was a shell of a man by the end of that (no pun intended)