Join a with a player who already has the A.J.M. 9 unlocked.
Since you are looking to unlock this specifically, here is an analysis of its performance to see if it fits your playstyle: far cry 4 ajm 9 unlock without uplay exclusive
While the A.J.M. 9 is a powerful early-game sidearm due to its high fire rate and suppressor capability, it is often outclassed by late-game pistols like the Join a with a player who already has the A