1. With as much energy as possible.  3. Going above and beyond the requirements, either for effect or to ensure one's effectiveness. Synonyms: (phrase) Going all out. Example: “Everyone counted Janice out of the tennis match due to her leg injury, but she rebounded quickly and really struck back with both barrels.  She even ended up winning the match.”

 

Etymology:

Based on the idea of shooting with a gun that has two barrels (tubes from which bullets are shot) which effectively empties the weapon of ammo.

See also: barrel, both