The mythbusters use a 7.62mm Dillon M134D minigun to chop down a tree in 45 seconds.
From Dillon Aero
The Dillon M134D Gatling Gun is the finest small caliber, defense suppression weapon available. It is a six barreled, electrically driven machine gun chambered in 7.62mm NATO and fires at a fixed rate of 3,000 shots per minute. Gatling Guns typically feed from a 3,000 or 4,400 round magazine. They are capable of long periods of continuous fire without threat or damage to the weapon making them an excellent choice for defensive suppression.
Dillon Guns are reliable. The M134D has system life in excess of one million rounds and an average time between stoppage of 30,000 rounds. In the unlikely event of a stoppage the weapon can be serviced and made operational again in under a minute. The multi barrel design means that each barrel only experiences a 500 round per minute rate of fire. This allows for repeated long bursts of fire and a barrel group life of 100,000 rounds.
Hat Tip: GizmodoRelated