If the ATF ruled that a shoestring turned a Garand into a machine gun, I’ve never understood how bump stocks made it through the process. Approving them was so out of character for the Obama administration, I think they may have just done it hoping that someone would use one in a massacre. It would certainly fit with the way they acted in Fast and Furious.
The difference is that the shoe string rig actually does make the gun full auto, as it uses it's own gas power (channeled via the op rod handle tied to the trigger) to release and re-pull the trigger each time. No need to manually release or pull the trigger each time with your finger, as with bump firing.
Bump firing amounts to nothing more than pulling and releasing the semi-auto trigger fast by a certain acquired skill in a technique, or using a device (e.g., bump stock, rubber band, or simple wood dowel) that minimizes the level of skill necessary to do it, but in every case, bump firing involves pulling, releasing, and pulling the trigger faster, with the finger, than is possible by the standard firing technique. That's not a full auto mechanism, and the stock doesn't make it a full auto mechanism, because the trigger still has to be released before it can be fired again.
