This guy made a super Nerf dart gun. He uses metal pipe as a barrel and hooks up an air tank to it.
In the video they show that the dart travels 80cm in 0.001 sec which comes out to 800 meters per second. Which comes out to Mach 2.35.
I was rather impressed with the amount of energy it has when shot at the plywood. I wonder if someone with a high speed camera could recreate this and see if the dart creates a sonic boom like those pictures we see of planes breaking the sound barrier.