Sounds like a .17 Mach 2 round to me.
http://www.hornady.com/story.php?s=145
It really doesn't pose a risk of discharging, but its clearly stated in the GC rules to not put live ammo in caches.
Finding and keeping unspent rounds from the range is not the best of ideas as you never know if they may or may not go off. It'd be like reloading and trying to fire a round that didn't go off the first time that the firing pin hit it.