This is a Barrel extension I designed for my NERF Stryfe, it has a clean outside and a decorative barrel rifling on the inside. Should fit other NERFs that support bayonet barrel extensions.
Designed in OnShape: https://cad.onshape.com/documents/fa3e34bb465a253d7817d529/w/8e7108d1335bbf100e561f57/e/f89d5c7e0345e26ca3927680
As there is a support with 0.2 mm air gap for easy break off, I strongly recommend using 0.2mm layer height.
Tip: Use concentric/circular top fill pattern to get a more machined look.
Break off the support
Use a pair of pliers to grip the inner circle in the bottom and break it off, it's about 12mm tall.
It's now ready to use. Note that the barrel is keyed (only fit one way).