It looks like it's rusty iron. Is it magnetic? If so, to me that rules out a bullet, unless it's a tip on something larger than small arms. I really don't think it's gun related, but at the same time, I don't have any ideas.
Minelab X Terra 705, Bounty Hunter Pro 505, Garrett Pro pointer, Pro swing 45, TDK WR 700 wireless headphones, leschee digger, green canvas mil surplus messenger bag as finds bag and brown bomb#2 to