The US has no armed drones that can operate at such high altitudes, and the UCAVs that do exist don't carry machine guns that could be employed against airborne targets. In theory it would be possible to design or modify such a drone but that would take a while.
Using guns against high-altitude balloons is also challenging just from an aerodynamics standpoint. Stall speeds get pretty high up around FL600 even with large wings, so the airplane would have to fly straight at the target with a high closure rate in order to get within effective gun range. This creates some risk of a mid-air collision. Using a guided missile is much safer.
Using guns against high-altitude balloons is also challenging just from an aerodynamics standpoint. Stall speeds get pretty high up around FL600 even with large wings, so the airplane would have to fly straight at the target with a high closure rate in order to get within effective gun range. This creates some risk of a mid-air collision. Using a guided missile is much safer.