May be a finial of some sort, but most of those for a guidon or flag were brass and attached to the wooden flag staff, by wrapping around the staff and were anchored by a brass rivets, tacks or screws. Your piece looks like it would be inserted into something. I thought it might be a tompion at first, but they're not often made of ferrous metals (at least those that I've dug weren't). Tompions of the period, again in my experience, don't have metal that would go into the bore either. Did you find, or do you know of, other military relics found at the site?