Did everything to buy it again from Kunar. Day 40th or something. Sleep, buy some stuff, sleep, buy , sleep. No chance after several days. The blunder-buss showed only once in Kunar's inventory in day 4th or so.
I also found it on the upper level of the umpani mountain. But I sold both of them to arnika arms dealer.I also tried to kill all the umpani but no gun yet ;(