As of Season 15, the medal is bugged, causing me to waste hours on end trying to get it.
What worked for me? In the Elimination game mode, getting 2 rapid kills. I first got the medal with the Huckleberry, but have since done it with other legendary SMG's, and even an uncommon. The kills DO have to be rapid kills however, hence most likely will be done with the same clip.
I believe the bug is that the game is giving players the "Double Play" medal, and not the Sub Machinist, ignoring the parameter for getting kills with an SMG, and just acknowledging the double kill.
Bungie... there are posts about this dating back to 2018... why is this still an issue?
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Got it this way: Huckleberry, hipfire playing control. About 10s betw kills