At 1 to 3 minutes before his most recent loot acquisition (1:40, if you're following the example), send 3 to 5 random dwellers into the wasteland.Once that time transpires, check your Wastelander's log to note the time when your dweller last received a loot item (for example, 1:43).
Send out a single dweller who has been trained well and is equipped to deal with the wasteland.You can easily obtain some truly epic loot by carefully following the steps indicated below, though note that it seems to work on only around every 1 out of every 3 attempts.