what common modern military equipment can survive 500+ years of continual use without becoming permanently unusable?
The only military equipment that can last unused 500 years is stone equipment: axes and arrow tips, oxidian blades and such.
[![enter image description here][1]][1]
Continuous usage would wear anything out in few years at best.
Unless the weapon is a bare throwing stone. [1]: https://i.sstatic.net/CEgBH.jpg