This is a NERF barrel extension sub-machine gun. (At least it's supposed to be the K) You can read more about it on Wikipedia: https://en.wikipedia.org/wiki/Heckler_%26_Koch_MP5#MP5K
This model is specially designed to work with the NERF "Stryfe", but it should fit other models.
Also included is a version for a front-piece/guard. This bit slides onto the rail on the bottom of the barrel and mates up with the rail on the bottom of this Stryfe. I enjoy using Talon2535's fore-grip handle with it: http://www.thingiverse.com/thing:815767
There's a "bayonet mount" ring bracket that lets you connect it to some NERF blasters. "Twist-lock". Some trimming of the connector may be necessary. Or you may need to add a few of layers of masking tape to get a tight fit, depends on the blaster. DO NOT FORCE CONNECTION or you may break your NERF blaster.
The accuracy of your shots might increase. But it might not.
I would like to get feedback.
Designed in OpenSCAD
Kossel Clear Delta
On my printer, using Cura to slice, I am able to publish this model with out any supports. There are some minor issues with the overhangs that wash. YOU probably want to use supports. I enjoy having a brim or a raft with my prints.