When the Cloud overtook the Sierra Madre, a security guard took to the armory right away, grabbing the biggest gun available as well as a sidearm and holed up in a building not far from the Police Station to wait for rescue. Unfortunately for him, the cloud started to seep through every crack and crevice of the room he was in, suffocating him under a thick blanket of toxic filth.

The Cloud was particularly intense in this small area, coating every nook and cranny of the firearms the Guard carried. This foul toxin bound itself to the metal, and by sheer luck didn't corrode anything too important, leaving these weapons tainted and fragile but functional.

Either due to the filthy clips and wheel of these weapons or the sheer amount of residue in the barrels, it coats the bullets in corrosive toxin, melting foes from within.

The Armor worn by the Security Guard was similarly soaked in the Cloud, weakening its condition and effectiveness, but imbuing it with a strange regenerative property similar to that of the Ghost People.

This corpse and both weapons can be found in a small room in a cloud filled area near the Villa police station, There's a tripwire just outside the door, so be weary, as it can launch the rifle if triggered. However I added a spare, less lore friendly copy of each of these to the "Sealed Sewer" to allow for it to be obtained after Dead Money, or possibly in DUST.

Goes well with my other Cloud themed weapon mods: 
https://www.nexusmods.com/newvegas/mods/80577
https://www.nexusmods.com/newvegas/mods/80590