Get the dart gun schematics and build it. (Uses radscorpion venom on the darts) (Know you know what those darts are laying around for )
http://fallout.wikia.com/wiki/Dart_GunThe Dart Gun fires darts that have been dipped in a radscorpion's poison gland. A hit from the dart gun will instantly cripple the target's legs, making it extremely useful against fast melee opponents like Deathclaws, though it seems to be less useful against Giant Radscorpions, even though their legs are crippled by the poison.
The poisonous darts inflict decent damage on the target for 8 seconds.
In short, its the gift that keeps on giving for 8 seconds. Smaller creatures will often die from a single hit.
Larger ones will be crippled, still moving, but drastically slowing their speed. Thus allowing you to easily stay back out of its reach and kill it with any weapon you choose. Personally I favor the flamer or the Pulse rifle, but that's just me.
Once I got the Dart gun it became #1 weapon for walking around the wasteland. Not great against robots and power armor. Slows everything else down nicely. Works a real treat vs FireAnts also. Cripple em, back up, head shoot them with whatever you have ammo for.
Underground in my opinion its hard to beat the combat shotgun.