Expanding on an idea I had before.
“Regular plasma guns explode all the time, and chaos plasma guns are regular plasma guns but even worse maintained.”
A few tweaks to their behavior. Basically, enemy plasma gunners should have an invisible overheat meter and the perk that gives your gun increased charge speed scaling with heat. They target a player and begin firing. The first two or three shots are slow to charge, but after that their charge rate and rate of fire increase quickly, provided the player they’re targeting stays in their line of sight. This would make them continue to be dangerous even against shields, both of the energy variety (psykers) as their damage output would rapidly increase after the first few shots, and the equipment variety (ogryn and arbites) as they’ll quickly drain the player’s stamina. This would also make them even more punishing against idiots who don’t take the hint and try to just keep dodge spamming while they’re getting blasted instead of getting the hell to cover. The increased rate of fire would not, however, make plasma gunners any better at hitting players who disengage and sprint away, so taking evasive action is still a viable solution when you’re targeted.
After the plasma gunner has fired a certain number of shots in the same barrage (say, around four or five maybe), their gun enters a Running Hot state. While in this state, the gun glows much brighter, emits smoke, and starts to beep. Also, the gunner’s armor rating is significantly reduced, which means while they’re blazing away they’re easier to kill.
But also, the best part: Every shot they fire while their gun is Running Hot has a chance to make them Overheat and explode, instantly killing the plasma gunner and probably also any other enemies next to them. The chance to Overheat further increases with each shot, maybe starting at around 25% and capping at like 75% after another three or four shots or something. This way you can never precisely predict when they’ll overheat, but it will usually happen within a certain range.
When the plasma gun overheats, the gun erupts in flames and violently shakes for about two to three seconds, before it explodes. Visually, this would be basically the same as when you shoot a flamer’s fuel tank a few times, with similar results, except an exploding plasma gunner doesn’t leave a pool of fire behind, and the initial explosion might do more damage to anyone standing close by.
An additional behavior: Plasma gunners won’t keep firing suicidally EVERY time, only most of the time, and only if they have a visible target to shoot at. While every shot they fire while their gun is Running Hot has a chance to make it Overheat, they also have a chance to basically suppress themselves; causing them to break off, retreat to cover, and reload, which resets their gun to normal. This is also beneficial to players, though not as satisfying as the gunner exploding, because it means the gunner stops attacking and has to reload his gun, which takes a really long time. Other events may increase the chance they’ll back off, like the target moving really far away, or if they’re suppressed or damaged.