How do you say firearm in Japanese?
— 火器

Related synonyms for firearm in Japanese
piece部分 (ぶぶん, bubun); 一切れ (ひときれ, hitokire); 一片 (いっぺん, ippen); 切り身 (きりみ, kirimi)
Specialized synonyms for firearm in Japanese
rifle(raifuru)