Some of you may remember the BATFE classified a shoestring as a machinegun a few years back subject to regulation under the NFA.
Maybe they have not had time to regulate rubber bands (yet).
The boys at Poor Man’s Machinegun explain it all to you.
John Jacob H's RKBA Commentary
A well regulated Militia being necessary to the security of a free state, the right of the People to keep and bear arms shall not be infringed.
John Jacob H's RKBA Commentary 