Here we go again....
"If we are to be realistic, having a letter from ATF saying this or that, carries as much weight as used toilet paper."
If this statement is true then why do people go around securing ATF approval letters when building a semi-auto firearm from a beltfeed machine gun parts set?
Just because a bunch of people go around doing something, does not mean that they are right.
"I will check around the ATF site on making semi-auto firearms from machine gun parts kits, maybe contact the tech branch about design approvals".
In addressing a citizens right to manufacture a home built weapon, federal law does not differentiate between one type or another of firearm. Only that the firearm built must be legal for that person to possess and have. There is no mention of how it is being made, or what parts are being utilized. A machine gun that has been demilled or otherwise referred to as a parts kit, is not a firearm, it is only a bunch of metal machined parts, hence, it is not a machine gun. It does not make any difference what it "used to be" at one time, if it has been properly demilled, eg. no receiver, then you can build it however the heck you want, as long as what you finally end up with, cannot fire more than one bullet with a single operation of the trigger, without manual reloading. The rest of the laws and rules that you must abide by are also clear, such as 922r, the parts count etc.
I can tell you, without any hesitation, that there is NO requirement for an individual to secure any type of letter or approval before building a home weapon, as long as the end result is not a machine gun. This is factual, and I am confident enough to repeat it again.
You should check out the federal regs to satisfy your hunger for knowledge, however, tread VERY carefully about calling up ATF tech branch or any other division and start asking too many questions. This is exactly what one fukcing dumb ass did regarding the Akins Accelerator, and we see what came of that. If an answer is already in front of you (if it is not stated in the regs, then that is your answer, it is not illegal), asking too many questions about it may end up stirring the pot enough to create the very thing which did not originally exist.