1200 rounds per second?! Are you sure about that? I can possibly imagine 1200 per minute, but 1200 rounds per seconds means that basically any size clip would be empty the moment you pulled the trigger.
Well, https://en.wikipedia.org/wiki/Metal_Storm No one said that it has to be in production....Metal Storm has created a 36-barreled stacked projectile machine gun, boasting the highest rate of fire in the world. The prototype weapon demonstrated a firing rate of just over 1 million rounds per minute for a 180-round burst of 0.01 seconds. Firing within 0.1 seconds from up to 1600 barrels (at maximum configuration) the gun claimed a maximum rate of fire of 1.62 million RPM and creating a dense wall (0.1 m between followup projectiles) of 24,000 projectiles.