Nerf Retaliator Barrel Spacer (Artifact, Worker, 17/32" brass)

This is a clone of the Artifact Retaliator barrel spacer that holds the barrels of the sealed breech kit. You glue the spacer in and then slide your barrel through (different tolerances may require some finagling on your end though).
The Artifact version is also compatible with the Worker Prophecy sealed breech kit, though you will have to install a grub screw, glue the barrel in, or something of the sort so it won't be pushed out (it has no knurling like Artifact).

The 17/32" brass version