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I'm putting together an exterminator using one of the forge-world turrets to complement an autocannon chimera for a small skirmish force of about 1,000pts.
The Exterminator will probably be the only tank in the force and the principal opponents are likely to be Orks & Dark Eldar.
Is the best option automatically 3 x HBolters & 1 x HStubber or is there a case for having the Hull Lascannon or a couple of HK missiles "just-in-case" something heavy crops up?
The rest of the "army" consists of squads with autocannons, a sentinel with autocannon and a missile launcher squad.
Only an idiot fights a war on two fronts. Only the heir to the throne of the kingdom of idiots would fight a war on twelve fronts.
I'd say you should go with the 3x Heavy Bolters, Turret TL Autocannon and the Heavy Stubber, anything else would be a waste of the Exterminators potential. It's my opinion that all types of Leman Russes should always try and maximize their infantry killing power. The reason why I give my Leman Russ Demolisher a lascannon is not to let it kill tanks more easily, it's to add an extra AP2 shot to hunt Hive Tyrants, Carnifexes, Obliterators or Termies.
Leave the tankhunting to your infanry or Sentinels.