Incorrect. If the receiver is going to be part of a short barreled rifle (SBR), it must have an ATF tax stamp to be assembled with the upper under 16" and that upper must match the spec of the authorized Form 1 parameters. As submitted, my Form 1 called out 30 caliber, 10.5" barrel on the lower. Specifically 300BLK.
ATF is very specific that applications for a "full auto" Form 1 or Form 4 will not be approved for new (post 1986) firearms. Pre-1986 full auto sold by a Class 3 FFL can be approved. There is lots of junk with high price tags that meets the "full auto" definition. Unless you just want to collect them, be very discerning in acquisitions.
You’re still talking about NFA firearm. That was not clear to me from your post.
For those NOT making NFA firearm, Form 1 would not apply.