I had the same problem. I believe they are only available to access when an Event is running.

How do I maximize Core Damage?

How the heck are some guys able to get 6%+ damage to the core in one attack? I was inflicting around 2% damage despite having a decent gun (Death Hazard maxed at 2130K), using Piercing Bullets, having Slow Time on the entire time, using Spotter to keep the diverting to kill snipers / drones / etc. to a minimum, and my clan had the bonus time!!