Definition of 闇夜に鉄砲
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(n) aimless attempt; shot in the dark
闇夜に鉄砲じゃあるまいし、そんな場当たり的なやり方でうまくいくとは思えないよ。
That's just a shot in the dark. How do you think you'll succeed by just acting on the spur of the moment like that?