The HK21 can be found in the Nazi Zombies game mode and is only obtained from the Mystery Box. It starts with a magazine size of 125 rounds and with an extra 500 rounds. When Pack-A-Punched, it becomes the "H115 Oscillator". This increases the magazine size to 150 rounds while increasing ammo capacity along with the usual additions.
The low rate of fire, very high magazine size and high reserve capacity makes ammo less of an issue for the HK21 than for other weapons. However, the very slow reload time makes Speed Cola useful when using this weapon. The low rate of fire also makes Double Tap Root Beer beneficial, but not a necessity. Be aware that Double Tap Root Beer increases the upwards recoil a significant amount.
Due to this weapon's high power and large magazine, it is arguably one of the best non-Wonder Weapons in Zombies mode.