This triumph has been bugged since its release in that it is super inconsistent. The smg's I've seen that seem to be the most effective at actually activating the medal are solar smg's, ikelos smg, and huckleberry. I wish it was a better answer for you, but it is basically just keep trying and hope it procs. I would recommend trying one of those weapons though since people have had most luck with them. Seems pretty unlikely that it will ever be fixed at this point since Bungie has never even acknowledged it.