The law rbmcmjr is talking about is the Gun Control Act of 1934. It required registration of all fully automatic weapons, silencers, short barreled rifles, short barreled shotguns, and destructive devices.
The registration process is actually quite simple. An ATF Form 4 must be filled out in duplicate along with 2 fingerprint cards, a citizenship form, 2 passport photos, and $200 (there is a $5 AOW transfer for some guns). The Chief Law enforcement officer requirement (which is NOT part of the law, but added by the ATF years later (absolute bull$hit)), can be circumvented by forming a coorporation and registering it under the corp. And, by the way, the CLEO signoff is meant to affirm that they are legal in the local area....not for permission.
All this can be done only in states that do not have additional laws restricting ownership, which, sadly, there are many (OK has no restrictions WHOOT).
The large problem with machineguns is that there is a limited supply do to the "Make My Day" law in 1986. The democrats tried to poison the law by adding a bit that restricted all future manufacture machineguns to .gov only. The NRA betrayed the lowly citizen and gave its blessing to the law despite the poison pill, and Reagan signed it.
Due to that law, there is a limited supply, as all machineguns that civilians can own had to be manufactured prior to 1986. This is why machineguns start at $3200 for a Mac, and go up to a half million for a minigun. Hell M16's are even around $13,000.
That law did not, however, affect the other item listed in the GCA 1934, so silencers, SBR's, & SBS's, are still cheap (relatively speaking).