A maker decided to see how fast he could get a Nerf dart to go. Apparently you can ratchet the velocity of these little foam darts up quite a bit. He ran it over the speed of sound apparently. 2.3 Mach. Pretty speedy. Take a look at the video.
The scary part is seeing it tested out against a board at close range. I can imagine some terrifying 3D printing add on's you could make up and stick on the front of the dart.