A couple days ago I measured the muzzle velocity of my modded Nerf Recon gun, it came out to about 75 feet per second, or ~51mph. That's really fast, although with a foam dart it'll slow down fast as well. I did some range testing last fall and the average range was about 75-90 feet.
Now you might be wondering how I went about measuring the muzzle velocity of a nerf gun? I did it with my webcam.
But not just any webcam, my modded PS3 Eye webcam, capable of recording video at up to 125 frames per second. I shot a dart exactly 3 feet, and measured how many frames it took to travel that far. I used 2 laser triggers of sorts, a green laser at the muzzle, and a red laser at 3 feet. I turned off the lights, and shot the dart between them. In the recorded video there was a green flash, followed by a red flash. The dart traveled 3 feet in 5 frames, or 5/125 frames per second = 1/25th of a second. Then I divided 3 feet by 1/25 and got 75 feet per second.
I've posted this and more fun physics on the forums of the company I bought some aftermarket parts for my nerf gun:
Stage 2 OMW Recon Muzzle Velocity
I've also posted an idea of mine, to convert a nerf gun into a semi and fully automatic nerf gun, using compressed air, a pneumatic cylinder, and some electronics:
The main purpose of the concept is to not change the firing power of the nerf gun with compressed air, only increase the firing rate.