With the change to weapon slots in the upcoming Forsaken DLC, lmgs could be viable since they could be put in any slot. I never used them in D1 (on my Xbox account) but if i could use a heavy machine gun, rocket launcher and sniper i think that would be pretty nifty. They should atleast be considered, an lmg would fit perfectly into the game when the weapon slots change.
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If rocket launchers can have a cluster-bomb perk then LMG's should have a perk that penetrates Knight shields.