I had that same question first time I picked up CPVC. CPVC is good for stephans (1/2" FBR). You CAN use it for regular darts if you dremel/file out the insides a little bit (just the front really), but 1/2" PVC is best for Nerf Micros
Find an O-ring at your local hardware store. And just personal opinion, but the best part about the AT2K isn't the range (though it's very nice) but it's the velocity of the dart. VROOM!!!!