Shotguns in pve has been neglected for so long and all that it is good for now is just for one two punch builds.
It did look like you tried to make them more viable when you added Chain reaction to them, but it wasn't enough. Even worse is that you nerfed Chain reaction just because you wanted to add it to more weapons.
Similar is the destabilizing rounds that causes void explosions. This one is much better than chain.
A simple damage buff is all that Shotguns need.
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easy fix : double their ammo reserves as somebody who often uses shotguns in pve, its the only special weapon type that basically forces you to use 1 bullet per ennemy (and often more than one) while i can use a fusion to clear multiple ennemies in one shot, same for glaunchers (and lets not talk about the ammo economy of rocket sidearms and trace riffles) only snipers have the same issue, but they are another problem pve wise tbh (and yes ik about combos like discord + destabilizing rounds, but it doesnt solve the shotgun problem)