So, I stumbled upon Scorpion Gulch last night with Boone and we were doing just fine... until one of the Giant Radscorpions got stuck on the rocks coming down a hill. Nothing unusual, of course, for Radscorpions to have graphical/terrain glitches (it's getting quite old, in fact). I thought nothing of it and starting exploring the area as Boone took pot shots at the unfortunate creature. A few minutes go by and I come back to find Boone STILL pumping round after round into the dirt around this same Giant Radscorpion. Sure enough, Boone's rounds appear to be passing right through the thing and kicking up dust behind it. I pull out my rifle and take aim: no hits. VATS? Nothing. All of the shooting mechanics appear to be working, but the creature appears etheral. His hit points have not changed in several minutes of Boone's assault.

I reloaded a recent save and retraced my steps from Helios, killing a number of scorpions along the way. Sure enough, when we get to the heart of the gulch, the same Giant Radscorpion comes down the hill and gets stuck. Same situation. I tried Grenades with no effect. Then, I got an idea and told Boone to switch to melee, at which point he ran up and started taking hit points off the scorpion with a Machete! So, I switched to a Nailboard and had the same results: this particular Giant Radscorpion was vulnerable only to melee.

I didn't have every possible weapon in the game with me, so I can't speak exhaustively about the permutations, but this thing was immune to my All American, Cowboy Repeater and Frag Grenades; as well as Boone's unique rifle.