Looking it up, Musashi had a shoulder injury, so that bullet is nice and mushed by now. Still, Jiro has something to work with!

He says it's not typical bulletsmith work, and after enough time examining it, he's going to start to freak out a little, namely because... these bullets have an alloy component he hasn't seen in a long time. It's Hihi'irokane, a legendary scarlet metal. Namely, it can conduct soul energy.