The XM-345 Skull Drill is a weapon featured exclusively in Thing-Thing 4.
An unusual weapon from Systems Corp., the XM-345 Skull Drill is a gun to be feared by those on the business end. It fires a small missile with a drill on its end, that homes in on an unlucky victim's head, drilling through it and then detonating.
While it is quite a potent weapon, there must be flaws to compensate for such deadly power; it only works if you aim directly at an enemy's head, the missile moves at a fairly slow speed, and it has a low rate of fire. If you fire the weapon and you're not locked on to an enemy's noggin, the missile would just stay there in place.
It can only be obtained from the weapon room in Genetics Lab 2. In addition, the weapon also comes with its own unique reticle.