Intent is key...
thats kinda stupid . i mean i have gun powder here , i have cannon fuze here , and i have pipe here ,...
True, but the powder, fuze and pipe can all be used for different things than just making a bomb. Full auto parts can only be used for making a full auto fun,.. er gun.
And thats enough to wind you up in court. Short barrel AR uppers are perfectly legal parts, and can even be bought direct mail order. BUT, if you have any AR lower receiver that the SBR upper can fit on, anywhere in your posession, you can count on facing prosecution, if caught. BATF doesn't always win "constructive posession" cases, but they do win a lot, and even if you avoid conviction, you have still lost a lot of time, MONEY, and maybe property.
Not even remotely worth the risk. Silencers are another example. You can have silencer parts, and be completely legal. BUT, if you have all the parts (even in unfinished form) you are at risk. Because the law says the crime is the intent to silence a firearm. You could build a silencer that doesn't work, and still be convicted, and if the jury believes you intended it to work, that conviction will stand. You don't even have to build the thing, if the prosecutor convinces the jury that you intended to build it (and why else would you have the parts, anyway?), you are going to Club Fed for at least a few years, and will never legally own a gun again.
Some full auto parts are, by and of themselves, legally machine guns. The post 1986 AR auto sear is the most famous example. Ownership of just that part, with no other AR parts or AR rifle, at all, can land you in legal trouble the same as having a functioning full auto rifle.
No, its not fair. No, it doesn't make sense. But it is the law. And until we change the law, it is what we have to live with. We are allowed to not like it. But we are not allowed to violate it, without facing the penalties.
Don't do it.