pelting rain; torrential rain; downpour
Vivid, somewhat colloquial term for rain that falls so hard it feels like bullets. Often used in spoken Japanese or informal writing.
Suddenly a pelting rain started and I got soaked.
鉄砲雨の中を歩くのは危ないよ。
It's dangerous to walk in such a torrential rain.
土砂降り is another common colloquial term for a downpour, but 鉄砲雨 specifically evokes the image of rain hitting like bullets.
Compound of 鉄砲 (teppō, 'gun') and 雨 (ame, 'rain'), literally 'gun rain'. The imagery is of rain falling so hard it feels like being shot. The exact origin is uncertain, but it is a vivid colloquial coinage.