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I'm slowly realizing that chaingunners with spider mastermind firepower are overpowered. If you don't see them (and have mediocre health/armour), you easily die.
What to do if the player chaingun is twice as fast as default, and these dudes carry two such chainguns?
Have them drop an ammo clip!
Another idea: Lunatic has these zombies that fire revenant missiles, and they have incredibly long delays between the beginning of their attack animation and when the rocket actually is fired. Frankly it's so long that it's REALLY funny, but I think you could look at it for comparison's sake and then pick the longest delay you can without crossing over into farcical territory.
I toyed around with the idea of a beefed-up chaingunner myself once. (And also an "SSG" zombie.) You could make potentially OP monsters work with careful staging. The Valiant pyro knight comes to mind; skillsaw mentioned that he only revealed them in front of the player, because of course getting wrecked by an unforeseen stream from behind would be quite unpleasant. So the analog here: you could favor placing these superchaingunners where the player basically can't miss them: close-quarters set pieces (with sufficient cover of course) where all reveals are obvious seem like a great spot, for one thing. I think in incidental placements I'd use them a lot more sparingly than a regular chaingunner, favoring areas with limited line of sight (meaning: it's unlikely that the player will unknowingly wake them up and have them roam aimlessly, and it's unlikely that the turret variety will be able to snipe the player from a distance).
I think a regular chaingunner is already a borderline mid-tier monster, so provided you don't give them a counterbalancing nerf (such as the aforementioned long delay between telegraphing and attack), it's probably a good idea to treat these monsters as, similar to a monster with ~500 HP in terms of numbers: use them in smaller groups at most, with larger groups saved for rare occasions.