To my knowledge, it's the most compact tubular magazine gun to date. These are the instructions to build and fire it.
To entice you to build, Insight also:
Can hold up to five bullets at a time.
Gets about 25 feet; this is very good for a pistol of this size.
Uses a red rod sized firing pin.
Has a comfortable handle.
Requires no cut pieces. (Nor do any of my posted guns, come to think of it.)
Shoots green rods. (A downside for some, but a pro for others).
And... Yes, the gun has a theme song: Insight by Joy Division.
Step 1: Build All The Things
Also of importance, those bands do not have to be cut and tied together; it would actually be better if you left them intact; I just ran out of full looped bands, sadly.