We’ve already done this. It uses a focused explosion force similar to rockets to dramatically increase the projectile’s speed, as well as allowing the use of smaller, denser, unfeathered projectiles more similar to crossbow bolts. Due to the much greater speed, said projectiles usually do much more damage than the arrows of old, and are much harder to dodge, as well as requiring much less training to use. We’ve fairly universally decided that the official english name of this new, explosion-powered projectile-throwing device will be a “gun”. 
This could actually work, but you’re talking about an absolute best force efficiency of 50%, since half of your force would just work by pushing the hammer away from whatever you just hit. And something like that would basically require the use of some type of explosives in the hammer head, nothing else would really be able to deliver the force you would need at the speed that you need it (and such a device would probably play hell with the hammer’s structure). Essentially what you would be doing is putting russian rocket boots onto the end of a stick and hitting people with it.
