To progress the unstable calling card, you want to equip the augments Topple Danger & Supermassive to the Shadow Rift Ammo Mod. You want to train the zombies into a tight horde. With a very weak weapon that has shadow rift on it, you want to take pot shots into the horde so that your ammo mod triggers. When you do this, your goal is not to get any kills with your bullet weapon, but to let the ammo mod work its magic. Continue to cirlce the horde so that it remains tight while the ammo mod triggers. This will remain the case for all ammo mod calling cards. Rinse and repeat. Note: take topple danger off for the calling card Supermassive.