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Heavy Flamer Gun
Like the Shishkebab, the Heavy Flamer Gun may not be the best in a tight combat situation or a closed room, but in an open field, let that baby rip. Balance it out by pairing it with a Gatling Laser or a Minigun.
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Gatling Laser Gun
If you're gonna do lasers, this is the way to go. Alternately, you could also use the Unstable Laser Gatling, however in exchange for increased damage it may suddenly break on you, a chance you may not be willing to take.
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In general Miniguns are always a great bet, especially if you're not the one having to lug them around. They tear through a crowd easily, provided your targets are of the soft skin variety, and pump enough rapid fire bullets that you can easily keep back waves of Ghouls. Also available is the Explosive Minigun, although like the Unstable Gatling Laser this one holds a lesser spot on the list despite its increased firepower, due to the possibility it may spontaneously break.
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While I recommend that most of the time you give your robot two different attachments complimenting each other's strengths and supplementing their weaknesses, you should definitely go double with Missile Launcher hands. According to my "research", a single Automatron with no body armor can take on a Deathclaw 1 on 1 and kill it within seconds. By far this is the best arm you could possibly weld onto a robot body.