Suppose, for the sake of hypothesis, that an individual has a prosthetic, mechanical hand, which he uses to fire a semi-auto sporting rifle; and that the forefinger of this hand has been made capable of pulling the trigger at a rate of 950 / per minute. Has the ATF ruled that such a finger would constitute a part of the trigger mechanism, or would it be considered separate from the firearm?

you'll need a form 4 fer that finger, pal...