This is a NERF barrel extension modeled after the WWII British Sterling sub-machine gun.
I've been designing a number of NERF barrel accessories, and the diameters on this model have been scaled so that a regular NERF dart will fit through the middle, and so that the outer diameter matches a cylinder that fits the outer diameter of certain NERF blaster barrels.
Yes, the cylinders at the tip are supposed to be off center.
There is a "bayonet mount" that lets you connect it to some NERF blasters. The added length might increase the accuracy of your shots, but it might add drag. Either way, it WILL look super cool.
DO NOT FORCE CONNECTION or you may break your NERF blaster.
I have made several other designs with various greebles.
I uploaded 5 files: one is the complete model, the others are the model broken into 4 parts for easier printing, but they will need to be glued together. PVC cement worked well for me.
The model is tall and skinny, and it could easily get knocked over.