On the other hand, a Machine Gun (a true one by definition and not an M16 style weapon) is an Assault Weapon, which is banned (without permit) by some law in the early 1900s. Technically, it's like .50 cal and bigger. So we're talking really big guns.
So having a 200 round drum is no big deal, since you can't own the gun that uses it (barring special permit of course)
A semi-automatic weapon (like an AR-15) is something you can own, because it is NOT an Assault Weapon. In which case I suppose that's where the concern about larger capacity magazines comes in, but that is only my non-political-intenting guess.
Other than the fact that M-16s are automatic weapons (or have an automatic mode) and automatics are also outlawed, I don't know the more about the actual law. Only that they don't fall into the actual Assault Weapons ban because that law covers it by size.
Wow...ok don't take offense, but I'm going to try to bring you up to speed on gun laws and how they apply to the AR-15 and the M16,
Ok, so first of all, an M16 is not an Assault Weapon, it is an Assault Rifle. You may ask what the difference is. An assault rifle is defined by a combination of cartridge and function. It is a magazine-fed selective fire weapon that fires an intermediate cartridge. On the other hand, an assault weapon is defined (in the states that define it at all since there is no longer a federal law that does so) primarily by cosmetic features that have nothing to do with its function.
The federal Assault Weapon Ban was passed in 1994. It had nothing to do with M16s (remember, those are assault rifles, not assault weapons). It banned magazine-fed semi-automatic weapons with two or more "military" features such as pistol grips or barrel shrouds (although why these are considered military features is something I still don't get). Note that caliber is not one of the criteria. That ban expired in 2004 and was not renewed. However, states that have AWBs generally have them written with AR-15s in mind (although there are variants that are legal in every state).
Automatic weapons (which I referred to as machine guns for the sake of ease for those here that are not overly familiar with firearms) were regulated by the National Firearms Act passed in 1934. It required that automatic weapons (as well as SBRs, SBSs, and suppressors, all of which I disagree with regulating so strictly, but that's another matter) be registered and that individuals obtain a federal license to own one. Note that there is no mention of the caliber of these weapons. In 1986 a law was passed that banned the registration of any more automatic weapons. Ones that were already registered can still be owned, but because of the ban they cost over $10k.
So yes M16s are difficult to obtain, but that still doesn't explain why it is ok to have a 200-round drum for a fully automatic weapon but not for a semi-automatic weapon. Automatic weapons are certainly more rare, but they could make better use of such large feeding devices. There are also other types of NFA firearms (I just used automatic weapons because they make the best example for how silly that law is). As ZB mentioned, you could also load it in an AR-15 with a short barrel (which is much cheaper and easier to obtain than an M16).
Also, I'm interested in whether or not the law applies to illegal NFA firearms (those that should be registered but are not). Interestingly enough, you cannot prosecute someone that illegally owns a NFA firearm for not registering it (as that would be self-incrimination which violates that individual's constitutional rights). So depending on the wording, it is possible that someone that owned an illegal automatic weapon could also legally load a high capacity magazine into that illegal weapon. Funny how the law works sometimes.
And again, neither law has anything to do with size. Caliber is not what makes an M16 an assault rifle vs assault weapon or why AR-15s are ok. AR-15s are legal because they are so versatile that they spawn variants that continue to bypass measures specifically intended to restrict them (which is aided by the fact that those measures are written/passed in extreme ignorance). M16s are (by and large) not legal because they are automatic weapons.