I was unaware of that ATF ruling, but the fact is that an FA selector would have no other purpose or use except in a selective fire rifle, and it is an integral part of those rifles. In the FA position, it allows the hammer to fall as long as the trigger is held back. Without the FA hammer and auto sear, the hammer will just follow the bolt carrier down, resulting in a misfire.
This.
As for owning the parts, ATF's position changes from time to time, agent to agent, but since your semi-auto receiver cannot accept the auto sear, they'd have a pretty tough time making a case for possession of MG or constructive possession.
FA parts are legal to sell, legal to own. You just can't own them at the same time you possess a receiver that can accept them. As for owning the semi-auto receiver, that's where it can get a little fuzzy. The receiver of any semi auto sold in the USA would require modification to accept FA parts (AR, AK and others), so it's not like you could just drop in a FA parts kit and rock'n'roll. I suppose they
could try to get you on constructive possession, but without a modified receiver that can take the parts, it'd be like trying to go for constructive possession on someone that owns any semi-auto and a decent selection of tools that would allow FA conversion with a little bench time.
I see title I dealers and private sellers at shows all the time with AR parts galore and FA parts kits spread out on the table, yet I've never seen them harassed by attending ATF agents. The reason they are left alone is that they DO NOT have a FA receiver that can take the parts. And, of course, having said receiver would be possession of an MG with or without the FA parts to go in it.
In short, I can't imagine there being any legal problem with using a commonly available FA selector, but it'd also be pretty pointless for the reason mentioned by JimK. Your selector would move to the FA position, but the gun would be a single shot in that mode, and one that ejects a live round in between each shot.
A pistol and a compatible stock, when added together give you a completely functional SBR, hence the charge of constructive possession.
Show me a case where someone went to prison on SBR constructive possession for having a pistol and a stock that would fit it, absent any
proof that the stock was actually installed on the gun.
That'd be like constructive possession for ownership of a rifle or shotgun and a hacksaw.