Barrels for the Nerf Knockout. Print with 2 perimeter walls and sand the internals if it's still too tight.

External fit is intended to be slightly loose for easier compatibility, either shim it up with some tape (I like masking tape) or epoxy it for a permanent solution.

Mega v2 has a tighter barrel fit, I find that the original has better performance but results may vary.

Knockout Inline allows you to load up to 3 half darts in the barrel. Use any 5/8" or 16mm diameter barrel material and chamfer the inside to aid loading. I use a 1.5" long CPVC barrel. Load darts via Knockout breech and fire. Then fire again. Then fire again.