Suppressors are no problem...in fact, if liberals were as concerned about the health effects of gun ownership as they purport to be, they would make suppressors widely available to cut down on noise, just as is the case in some countries in Europe.
The National Firearms Act was passed in 1938. My understanding is that the meaning of the Second Amendment was quite clear to the Federal government and that Congress recognized it could not ban automatic weapons. So it just put a tax on their ownership which, thankfully, has not been increased since then. Of course, the price of machine guns has skyrocketed since the pool of available weapons is relatively fixed. I say relatively, because every so often a few new machine guns enter the market...Vector Arms in Utah is selling newly manufactured UZI subguns, because they bought several thousand blank pieces of sheet metal which were registered as machine gun receivers before 1986, and they have finished milling them and building new guns.