The Defense Advance Research Projects Agency (DARPA) wants to build a missile with a gun on the the end of it. As the DARPA expense budget document (page 194) describes it:
The Gunslinger program will develop and demonstrate technologies to enable an air-launched tactical range missile system capable of multi-mission support. This system will utilize the high maneuverability of a missile system coupled with a gun system capable of scalable effects and engagement of multiple targets.
These mission sets addressed will include counter insurgency (COIN) operations, close air support (CAS) and air-to-air engagements. The metrics associated with this system include total range (which includes transit to target, loiter and engagement) and weapon system effectiveness.
The program will address the system and technology issues required to enable development of a robust missile system considering (1) vehicle concepts possessing the required aerodynamic, propulsion, and payload capacity for a wide operational envelope, (2) the algorithms that support maneuvering and target recognition to enable expedited command decision making for selecting and engaging targets and (3) approaches to incorporating modularity of design to reduce cost throughout the design and development process.
The anticipated transition partners for this effort are the Air Force and the Navy.
The idea seems to be that the Gunslinger would be capable of taking out multiple targets, either on the ground or in the air, while allowing a manned aircraft to stay safely out of range of hostile fire.
Exactly what kind of gun the Gunslinger missile system would carry is yet to be determined.