I hate to nitpick here...
I hate to nitpick here, but full-automatic firearms, short barreled rifles and shotguns, sound suppressors and any other weapons are NOT illegal weapons. They are, however, items that are UNLAWFUL to possess or manufacture without the proper tax stamp paper from the BATFEags and the laws to allow them to be possessed in one's state of domicile. I will not bring up the illogical NFA of 1934 and the 1968 act, the 1986 and 1989 acts and the now (THANKFULLY) sunsetted AWB (farce) of 1994 which serve no useful purpose but to harrass the otherwise sovereign and moral citizens of our nation.
As a gunsmith and gun shop owner I get asked a lot of these same questions and I do my best to explain it in terms that are easy to understand. I may use this thread to give visual explanations to some of them that need visual cues to get it to sink in.
I will state that you really, really, really have to be careful regarding pistols produced from receivers generally used for long guns, that is, AR-15, AR-10, AK series, FAL and such because if you don't you can get deep in a manure pile real quick. I have noticed that some manufacturers making AR-15 pistols mark the receivers, completed or stripped, with the marking "PISTOL ONLY" on the side and indicated they are for use as handguns only. You may want to use one of those receivers for a AR-15 pattern pistol rather than trust that a generic receiver will be accepted as a pistol rather than a SBR without paperwork. Trust me when I say that the BATFEags have NO sense of humor.
Always research something you are uncertain about because federal, state, county and municipal laws can come back to bite you if you don't know what they are beforehand. Though on Natural Rights and moral grounds I oppose any gun control dogma, I play by the rules because I don't want to be PC Bubba's down low doggy.