Thanks, A-a-T. I must have missed that, or forgotten about it.
Would you care to be more specific as to additional machining required to complete the fire control cavity upon removal of the biscuit?
Well, you can't just "remove" the biscuit. It has to be machined out (or attacked with an unregistered dremel, as suggested by EP Armory, if you want an ugly pile of unusable crap).
Once the block/biscuit is gone, you have to open it up by:
(based on my EP lowers - others may differ)
FCG pocket width @ trigger/hammer - approximately 0.2" (from about 0.5" to 0.690")
FCG pocket width @ selector - approximately 0.060" (from about .375" to 0.438")
FCG pocket length - approximately 0.350" (from about 2.200" to 2.560")
FCG pocket depth - approximately 0.150" (from about 1.1" to 1.249")*
Trigger hole in floor of FCG pocket - approximately 0.130" x 0.490" (needs to be 0.312" x 0.680" - starts at about 0.188" x 0.188", off center)
It has no real bearing on the current legality discussion. But as I've mentioned before, the trigger, hammer, and selector holes will be in the wrong places if you use the "indexes" (drill guides) to drill your holes. To end up with a functional receiver, you need to remove the guides and layout the hole positions yourself. (I knocked them off with a chisel.)
*(Because of the poor design, engineering, and wrong dimensions all over the receiver, milling to full depth greatly weakens the receiver, because the floor ends up quite thin. ...But your FCG and selector won't function without doing it - even if you layout the holes in the
correct positions....
)