one solution would be to go into Vault of Glass when it comes back around to being the featured raid and do the challenges to hope to get a exotic version of atheon's epilogue (void damage high rate of fire primary auto rifle) or go for the exotic word of crota from crota's end when it becomes the featured raid (void damage hand cannon with explosive rounds). Or if you are not playing in something with level advantages disabled, do the old versions of the raids and hope to get the old versions of them that are non exotic but still have the burns. problem with those are they will be weaker than newer guns and still require going through a raid. though that requires both luck and dealing with raid groups. Another possible solution would be to find a side are or other secondary weapon that always respawns with ammo, though there isn't many of those and getting one with a void burn on it can be a chore in itself. really wish bungie would of made all exotic quests complete-able in both PvP and PvE instead of either all PvP, all PvE, or requiring flip flopping back and forth... oh well hopefully in destiny 2 it'll be better.