Okay. It sounds to me, that the trigger assembly has a spring that pushes the forward end of the trigger up, in turn, pushing trigger back down.
My thinking on how I was gonna do it:
The part of the trigger that trips the sear has a inverted ramp on either side. It points toward the back of the weapon. I would assume in the FA gun, when you hold the trigger down, that ramp catches the sear and trips it again. I'm thinking it that if I modify that part to have just flat wings, then when the bolt and sear come forward, the sear can't pick up the trigger, if it is held down, so no firing and thus, no firing unless the trigger is released. I made a sketch of it at www.2loose.com/1919a4.jpg
. What do you think?