Bust FCG mod for RLL select fire is the easy part, and the hard part is finding a lower receiver to be the Host receiver instead.
Note, use a mill so it does not look like a basement hack joob with dremil to the FCG parts.

But, there are only a few low shelf receivers being offered, and then you get into if the receiver has the stop tab on the side that will allow the selector to be switch to full auto, and when the RLL is installed into the tripped receiver, that you can pull the Link all the way back to touch the back cut wall of the shelve cut out wall before the front round part of the back link will bind out on main receiver void walls isntead.

So if your start with one of the $50 Anderson lower receivers that has the correct selector stop tab locations for the three needed selector positions,

Your good on the outside of the receiver, but since its a high shelf receiver, then you have to mill the back shelf down to turn it into a low shelf receiver.  Also, since the RLL was right hand shaped for a SP1 receiver, you may have to mill the back walls of the trigger void areas as show above in the other post, so the back tail of the RLL can travel back all the way to touch the back shelf wall.

In Chris's photo from Quarterbore sight, you can see that it's boarder line if the Rll in that receiver will be able to travel all the way back to touch the shelf back wall before biding out the back FCG cut out section isntead.  Hence link will work fine to pull the disco back , but since the paddle tends to over pull the link backwards in actual full auto cycle, we do want the Lightning Link Re-Enforcer back paddle section to be able to tag the shelf back wall, before the front round section binds out on the back of FCG void radius walls.



As for ones you have all that worked out, then is just a mater of smithing/shimming the FCG for a clean crimp semi auto trigger break to  3lbs with zero creep, as well as adjust the disco timing so you don't end up with hammer follow through with the hammer and triggers sears squared, milled and polished in the semi auto function.  So set screw up the pistol grip channel solves any pre creep from selector on safe when trigger is pulled, and regarding trigger over travel, since the M16 trigger does not have a back wall, you have to off set the over travel after hammer break seat screw to pick up one of the trigger side walls for adjustment isntead.

You can piece out the FCG parts, but make sure that the hammer you get is a modified M16A3 hammer.

Hence auto sear tab on it ground off, but you need to make sure that the rear disco sear has the edge to the right surface.
 Rear disco sear like this is not going to work,


This is the hammer you need, since it has the extra edge on the right hand side side of the disco sear to work with the mod'd trigger burst cam instead.


As for trigger work, it mostly smitting of the sear to get the crisp break, a few take up bolt to solve take up and over travel, and since very few have the needed mills or talent so set the sears correctly in the first place, I tend to not post a "how to's for this work on the fourms isntead.  If you want to PM with a name and phone number, and talk you through the process if you are a machinist/have the tallent on the mill to do such needed precise work.

Short version, we set up the FCG to remove the  extra take up, over travel after hammer release to begin with with allen set screws, shim out side walk of the hammer and trigger to ID of receiver down to .001", set the engagement of the trigger to hammer primary sear at .020" square, then re-time the semi disco's for last of trigger release back to home postion for hammer release/reset in semi, then time the full auto disco to work with the RLL. Also, some light work on polishing the FCG pins and the ID channel of the FCG that they are going to ride in, and this will bring the trigger down to 2.5lbs every time.