Vietnam era Mini gunship/light cargo aircraft:
https://en.wikipedia.org/wiki/Helio_AU-24_Stallion
It packs a 20mm tribarrel on the door and five external hardpoints, but can also be used as a light transport aircraft and is optimized for STOL. It also mostly looks like a typical civilian monoplane so wouldn't raise any eyebrows at first glance.
Similar plane with the same style door gun:
https://en.wikipedia.org/wiki/Fairch...-23_Peacemaker