This weapon have only been seen in use by the US military forces trying to kill the undead in panama city in a flashback that Max had earlier in the story it´s unkown what happened to this weapon as it haven´t been seen since the biggining of the outbreak. A theory it´s that maybe when the US military forces destroyed the Panama city with explosive maybe they were destroyed in the process, another theory is that there may be more survivor that took this weapons, and the last theory is that Juan´s group know this weapon will take a lot of space in the Humbee were they keep there weapons.