弹 (bullet/bomb) + 坑 (pit) — literally "bomb-pit". 弹 reads dàn ("projectile, bomb") not tán ("to spring, to play [an instrument]"). The same character splits across two completely different domains: explosives and music.
mistakes
Don't read 弹 as tán here. As a noun for projectiles or explosives, it's always dàn — 子弹 (zǐdàn, bullet), 炸弹 (zhàdàn, bomb), 导弹 (dǎodàn, missile).
