Platinum GM run will drop one every time.

i got the slammer from my 7th guardian games hero nightfall (arms dealer). score doesn't matter. because my teammates were just melting everything as soon as they spawned. none of this emote finish stuff. and i know there were even yellow bars that despawned because they advanced to the next section before killing them.

It does drop from GG Nightfall as i got one yesterday. I never seen it before had to go check where it came from. Mind you i ran a lot of nightfalls previous weeks without a drop so was due something. I find one has to do about 11 runs at hero level before a drop then averages about 7-9 after that. Also helps if you get a Platinium score (all champs) , bit a catch on the Neomuna one if you kill the Tormentor to quick you miss an unstoppable champ.

Do the guardian games nightfall, hopefully the next 2 are better than hypernet, go for high scores, I got tonnes of the grenade launcher last week from 200k & 250k+ runs. Platinum helps, kill all champions