The Myth that Automatic Firearms are Banned - Actually, they're not banned at the federal level. Municipalities or states may ban ownership and use of fully-automatic firearms, but otherwise they are legal for private citizens to own and use. However, they are restricted. Since the National Firearms Act of 1934/68, their importation and sale has been regulated with a $200 tax on each item. Nowadays, this doesn't seem like such a big deal (considering the normal price of a rifle), but keep in mind that this $200 tax has not changed since 1934 - the cost back then was so prohibitive it might as well been an outright ban. In fact, one of my goals in this life is to own a fully-automatic, suppressed MP5. Silencers (more properly, suppressors), as well as short-barreled rifles/shotguns are also subject to this $200 tax.

The Myth of "High-Powered" "Assault" Rifles - Whenever you see the words "high-powered" "assault" and "rifle" together in a sentence, you can be sure that the author is either ignorant about or biased against guns. They almost invariably proceed to claim that such rifles are a menace to society, and can kill cops wearing body armor. The problem with their description is that just about any rifle greater than a .22 caliber is capable of defeating body armor. Might as well classify most peoples' cherished deer rifles as "high-powered." The "assault" adjective is also misleading - usually it just means that the particular rifle platform has a military look to it. "Assault" means it looks scary - the basic functionality of the weapon is unchanged. There is practically no difference between an assault rifle and any other rifle.

The Myth of the "Gun Show Loophole" - Most people would call less government regulation a feature, rather than a loophole. What the "loophole" amounts to is that the government has no control over private sales between private citizens. You might as well use the term "garage sale loophole". For some reason, when new firearms are sold, or any firearm is sold across state lines, the government feels it needs to know about it. But beyond that, they don't need to know. Private sales cannot be regulated, nor should they be. However, this does not let the private seller off the hook completely - they are responsible for making sure that the buyer is legally able to own a firearm before they can sell it. There is nothing official they have to do, but at the very least, most sellers will have some sort of CYA form for the buyer to sign stating that the buyer is legally able.

The Myth that the 2nd Amendment Only Applies to Militia - This is still a major point of contention in contemporary times, although I can't see why. In the interests of brevity and time, I'll just post the following quotes, along with the most salient posts:

U.S. v. Cruikshank (1875)

The second and tenth counts are equally defective. The right there specified is that of 'bearing arms for a lawful purpose.' This is not a right granted by the Constitution. Neither is it in any manner dependent upon that instrument for its existence. The second amendment declares that it shall not be infringed; but this, as has been seen, means no more than that it shall not be infringed by Congress. This is one of the amendments that has no other effect than to restrict the powers of the national government, leaving the people to look for their protection against any violation by their fellow-citizens of the rights it recognizes, to what is called, in The City of New York v. Miln, 11 Pet. 139, the 'powers which relate to merely municipal legislation, or what was, perhaps, more properly called internal police,' 'not surrendered or restrained' by the Constituton of the United States.

Presser v. Illinois (1886)

It is undoubtedly true that all citizens capable of bearing arms constitute the reserved military force or reserve militia of the United States as well as of the states, and, in view of this prerogative of the general government, as well as of its general powers, the states cannot, even laying the constitutional provision in question out of view, prohibit the people from keeping and bearing arms, so as to deprive the United States of their rightful resource for maintaining the public security, and disable the people from performing their duty to the general government.

Laurence Tribe in American Constitutional Law

Perhaps the most accurate conclusion one can reach with any confidence is that the core meaning of the Second Amendment is a populist / republican / federalism one: Its central object is to arm 'We the People' so that ordinary citizens can participate in the collective defense of their community and their state. But it does so not through directly protecting a right on the part of states or other collectivities, assertable by them against the federal government, to arm the populace as they see fit. Rather the amendment achieves its central purpose by assuring that the federal government may not disarm individual citizens without some unusually strong justification consistent with the authority of the states to organize their own militias. That assurance in turn is provided through recognizing a right (admittedly of uncertain scope) on the part of individuals to possess and use firearms in the defense of themselves and their homes -- not a right to hunt for game, quite clearly, and certainly not a right to employ firearms to commit aggressive acts against other persons -- a right that directly limits action by Congress or by the Executive Branch and may well, in addition, be among the privileges or immunities of United States citizens protected by §1 of the Fourteenth Amendment against state or local government action.

Some may say that it is hard to understand the meaning of the words written 200+ years ago - but it becomes much easier to understand when you look at the contemporary writings, such as the court decisions from only 100 years after - and those are worded much more plainly.
The above quotes were blatantly stolen from Kevin @ The Smallest Minority

Other considerations:

• The Bill of Rights (where the 2nd Amendment resides) are restrictions placed upon the government. Note the direction of power here - they do not give the government powers - rather, it takes power away from the government.
• The Bill of Rights does not grant powers to the individual - it assumes the individual has these powers already, and guarantees that the government cannot take them away.
• Each of the amendments applies to an individual person, or multiple individuals. The usage of "the people" is not in the singular collective sense, but rather in the multiple individual sense. For instance, the right of free speech does not apply to states, nor people groups, but rather all individual persons.

Got more myths for me to refute? Send'em my way, i'll cover them in the next installment!