>>64381201
BRN4 quality sort of mid depending on the parts. Ive seen uppers that look like they were carved with a dull chisel among parts that looked fine.
>Slightly longer FRT
MR556 BCG lacks the auto trip, you can get a trip extension but its always going to be jank. The MR556 barrel has a tab that comes off and rejects anything but an MR556 carrier to prevent you from swapping in an auto BCG. Hence the pull barrel, trim tab, swap carrier and boom actual sear trip. The BRN4 carriers have the cut for the tab and are sear cut so you could swap one of those in with your HK bolt and be a lot less work.

Ontop of this you will likely need to punch out the firing pin safety since most FRTs have fucked up hammer profiles. But thats going to be FRT specific.