I was poorly educated on guns and law as a child. Sadly I am not the only one. If you clicked this link out of curiosity and you already know the answer, then this is not meant to insult you, but it is meant to help those like me:
For the seriously lazy, just know that if a part or rifle on a website says “non-NFA”, that is good. You don’t need any federally caused delay or increase in cost to own that item. If it says “NFA” then it is going to be a while before you can own it legally, with the exception of a sub 16" barrel/upper. You can order those parts, you just can't put them on a rifle lower until it is registered as a Short Barreled Rifle (SBR). However, simply owning a short upper and a non-registered lower can cause you trouble with the ATF and there have been reports of people being charged for such a thing. Just because you can buy a short barrel before your lower is legal does not mean that you should. A safer bet for the less patient is to own a pistol lower (which has its own pros and cons) which will allow you to legally use a short upper.
NFA stands for National Firearms Act. Enacted in 1934 it was intended to control/affect the manufacture, sale and transfer of certain weapons. There are 2 important points to note: The first is that not all weapons are effected by the NFA. The second is that all weapons affected by the NFA incur a $200 tax, to be paid to the ATF. Fun fact: The $200 tax has not increased since 1934.
While the intent of the law back then was to control prohibition-gangster style weapons with a heavy tax, it has been modified somewhat from then until now with other acts like the Gun Control Act. And the supreme court decision of Haynes vs United States, where a felon used the 5th amendment to protect himself from registering under the NFA which would have required self incrimination. If you have filled out a form at an FFL recently you would know that particular loophole has been closed.
What this means to us is that auto weapons (more than one round per trigger pull), rifles with less than 16inch barrels, smoothbore shotguns with less than 18inch barrels, pistols with a vertical foregrip, grenades and a bunch of other random stuff are regulated under the NFA. The NFA does not prohibit you from owning such things, but it requires more regulation(and, don't forget - $200 per item) to do so. There are a few options, like a gun trust (which can vary from state to state and has some other benefits) to make fulfilling the regulations easier (and cheaper over time), but there is no dodging the $200.