My thoughts as well. Single-shot .22 short or better yet a small explosive charge (like 1/8 or 1/4 ounce HE). A quick get up close and personal, then self-destruct on command - an air-borne suicide bomber.
A YouTube demo HERE of formation flying - thought it was neat when they went through a "window" (at 1:08) - first thought was the above scenario.
I do like automatic fire for the assassin mission better than the HE suicide mission.
With automatic fire the assassin can fire several quick burst in to the target and verify the kill. With a self-destruct mission the assassin would have to have an observer or backup drone to verify the kill.
I think a .22 LR or 0.17 HMR Magnum would be my choice for a assassin drone for minimizing weight and maximizing fire power.
A two drone mission would be good idea. One drone could spot the target from say 3000 feet at a distance while the assassin drone could be launched from close range knowing approximately where the target was located.
A third armed drone could be held in reserve in case of failure by the first.