My grandson has a ton of Nerf guns that shoot darts. We just bought him a Nerf gun that shoots the Nerf balls (if you haven't tried one of these guns, they hit way harder). I wanted him to have a good set of targets that were strong enough to handle the balls while still being fine with just the darts. I always find a use for wallyworld arrows, don't ask me why... :p

Anyway, I drew this up in Blender. The holes in the spinners are elongated to help the targets want to stay vertical when they reset after a shot. In the picture, I only used two of the 3 (inch) and one of the 4 (inch) spinners. You also need two spacers per spinner (Elmer's Purple Glue holds the spacers where you want them). Two brackets for the ends, 5 shafts and 2 preferably old CDs or DVDs per spinner (glued in). I covered all the CDs on both sides with blank CD labels and my grandson painted all the centers of the labels red to make the bullseyes.This time the shafts I procured were .292" diameter, so I made all the holes .310". Scale your parts accordingly.

Edit 8/23/2017: Now that we've played with this enough... The current design is great for any of the Nerf Darts. If you're using the Nerf Balls, put 3 layers of CD Labels on each side. If you don't, they'll break the CDs pretty quick. Those balls are just too tough for the CD/DVDs even with a layer of paper on both sides. So... I'm doing a remix design that won't break (so easily) with the Nerf Balls.