The Ex-B (Explosive-bolt) is apparently an alien experiment in equipping the guardian drone model with a cannon that fires energy balls that explode on impact, instead of bouncing around. The Ex-B has a very long range if you manually angle the shot correctly. For example, you can be well outside Evergreen Mills and still hit the trapped super mutant behemoth. The Ex-B can also work underwater.
The drone cannon is an ineffective V.A.T.S. weapon, as the fired projectile will arc towards its target; since the game doesn't compensate for gravity in V.A.T.S., this can leave the projectile several meters away from its intended target.
In the weapons lab, there is a room where you can spawn brahmin for the drones to kill. Enter the room and kill the drone for the weapon. There is also a second one in the same room that is automatically activated when the first is destroyed.
xbox360ps3pc When shot, the projectile will occasionally pass through walls or the floor and do nothing. [verified]
xbox360ps3pc In some accounts, when shot, the projectile will just disappear as soon as it hits the target, damaging it but not exploding. [verified]
xbox360pc When shot into deep water, the projectile will disappear, and then reappear and seemingly "float" in place on the surface without exploding. Projectiles fired onto the floating projectile will attach and stack, also without exploding. [verified]
xbox360ps3pc In V.A.T.S. mode, the Ex-B seems to aim like a normal drone cannon, landing short of the enemy as if expecting the projectile to bounce. [verified]
ps3 On some occasions, the cannon will fire the projectile but will bounce off surfaces and not explode. [verified]