The Sentry Drone Gun is an upgrade available for the AmericanSentry Drone that equips it with a 20mm autocannon. This made the Sentry Drone a rather potent weapon against infantry, made deadlier by the drone's inherent invisibility. With these two traits, in addition to the Drone's high speed, they were highly effective at tracking down and eliminating stealthed enemy infantry - including Black Lotus and Jarmen Kell, both of whom were incapable of damaging them.
The 20mm autocannon itself resembles the M85 .50 calibre AFV turret-mounted heavy machine gun due to the distinct heat shield and flash-suppressor. However the size and length of the gun do not correspond to the actual real-life M85, perhaps hinting that it is an auto-cannon with certain elements taken from the M85.