This seems like a very literal interpretation of a boomstick.

Backblast is Newton's third law. The blast out the back balances out the force of the projectile going forward. Otherwise you'd have to have a bolt/breech with sufficient mass and strength to contain the pressure and recoil back. Intuition tells me the balance isn't perfect and there will still be some recoil force, hence why the launcher is still flying off the guy's shoulder. He probably didn't have a steady enough grip.