My Lower that I used is a Spikes Tactical Honey Badger Lower (saw one and had to have one), so I bought one and Checked it for in spec and then Sent in Stamp Paperwork for my 300 Black Out Build.
Now as far as other parts, I am not in a Hurry to get done but been buying used parts and a few new ones here and there (of course when on sale).
I got a used Mega Upper and put my 10.5 Barrel on it and it looks SWEET.
Paid 60.00 for a complete used Mega Upper (no barrel) and now thinking that is what upper I will use for my build.
Bought a Cheaper Trigger group will upgrade later if needed, the one I got is a Advanced Combat Trigger (60 bucks).
Still looking at Rails as I have not decided which one I want.
Bought an ARC30 Suppressor from Dobbs Defense for this build.