Here's something I put together over the last few days, still far from finished but as of now it's in a functioning state.

This blaster was designed for taking out long range targets in Humans Vs. Zombies, NOT nerf wars.
It uses what I call "assisted bolt-action." It's best described as the halfway point between bolt-action and semi-automatic. A fraction of a second after firing, the bolt opens. Due to the non-resetting trigger, the bolt stays open to allow the player to load a new dart. The trigger is then manually reset by pushing it forward which in turn closes the bolt and advances the dart. Because the action of pulling and manually resetting the trigger is so minimal compared to cycling a bolt, I call the process "assisted". The player never manipulates the bolt directly, only the trigger valve.
As you can see I have yet to install the actual trigger to the valve, but that'll be done soon. I still have to clean it all up a bit and then give it a healthy makeover with bright colored duct tape so it looks ridiculous as a nerf blaster should. More to come...