There's only really two ways you can get 1hit killed versus a Jackhammer.
1: Run right into it through a door. you're running for the control console, you open the door... and you're staring the gun in it's three-barreled face. your next thought is usually 'shit.' and your next action is most likely 'fall over dead'.
2: Get snuck up on. I cannot tell you how often i've snuck up on people and MAXes with my JH. i can tell you that it is a rude awakening, because i can get right up next to you and triple-blast you.
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