Here, found it..
http://www.atf.gov/publications/download/p/atf-p-5320-8/atf-p-5320-8.pdf
and in that on page 21.
2.1.6 Machine guns.
It specifically states per single function of the trigger.
Pull and release are two separate functions of the trigger. You can modify a gun trigger to fire a shot with a pull, and a release of the trigger. It is legal.
This is why there is release triggers and pull triggers that fire on the release or pull. You can modify a gun to fire for both though since each is a separate function of the trigger movement and actuation.
Good day.
You need to re-read that section.
“designed to shoot automatically
more than one shot without manual reloading by a single function
of the trigger”
Reloading is the release you are talking about, we call it trigger reset.
2.1.6 Machinegun. Firearms within the definition of machinegun include weapons that shoot, are
designed to shoot, or can be readily restored to shoot, automatically more than one shot without manual
reloading by a single function of the trigger.
Included within the definition of machinegun is any part designed and intended solely and exclusively,
or combination of parts designed and intended, for use in converting a weapon into a machinegun. This
portion of the machinegun definition addresses what are commonly referred to as conversion kits. The
“any part designed and intended solely and exclusively” language refers to a part that was produced for
no other reason than to convert a weapon into a machinegun. Illustrated below are examples of such
parts.
I'd still like you to explain how a trigger group is going to fire another round during reset, and if possible show me an example of such a firearm.