The military plans to test the Advanced Tactical Laser, a laser weapon mounted on a C-130H air transport that could destroy any weapon system without collateral damage. . . . Under the program, Boeing would test the aircraft in July 2006. The aircraft would have all subsystems on board except the high-energy laser. Officials said a low-power surrogate laser would be used instead of the kilowatt-class, high-energy laser. . . . The ATL was deemed as complementary to the Airborne Laser program for the Missile Defense Agency. ABL was meant to mount a megawatt class chemical laser on a Boeing 747-400 freighter aircraft.
A C-130 is a 1950s technology, and both low powered lasers and C-130s with weapons on them are 1970s technologies. But, efforts to built high energy lasers have not been very successful. The existing efforts have been over budget and produce massive amounts of toxic waste and are hard to get small enough.
Also, for clarification purposes, the idea, apparently, is to heat enemy ammunition, causing it to explode on its own, rather than to have a science fiction style laser gun. The obvious countermeasures would be to start using railguns, which can have inert ammunition, or to create some form of artificial fog which would diffuse laser light.