Rifle Bullet, Bullet
noun
A gun's bullet is a bullet, though this word can specifically refer to a rifle bullet as well (probably because it was mostly rifles that people had way back in the day).
じゅうだん
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
少し待って貰ってもいいですか？ちょっと銃弾を取りに行かなくてはならないので。
Could you hang on a second? I have to get my bullets.