Summary

Nerf Barrel Coupler Test (For making a variety of barrels).

Base models from the following different couplers...

(1) "Nerf Barrel Extension Adapter" (http://www.thingiverse.com/thing:41914)
(2) "Nerf Dracogen barrel extension" (http://www.thingiverse.com/thing:451186)
(3) "NERF barrel extension (#1) -- basic design" (http://www.thingiverse.com/thing:383058)

From what I can see, I think I like #3 best :)

Test files on tinkercade: https://tinkercad.com/things/9OP9chOQpgG