> 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]

[![enter image description here][2]][2]

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
  [2]: https://i.sstatic.net/3cCiZ.jpg