This lets them fire on both barrels

This is a nice subtle malaphor, conflating “with both barrels” (with as much energy as possible) and “fire on all cylinders” (to function as efficient as possible).  Both idioms are similar in meaning, and both involve combustion. “Both” and “all” might also be contributing to the confusion.  In context, the writer was discussing certain marketers that produce content efficiently but also are audience-focused.  Hence “this lets them fire on both barrels: quantity and quality.”  A big thanks to Marcia Riefer Johnston for sending this one in!  You can boost your writing skills by visiting her wonderful website at http://writing.rocks.