When you see someone posting nonsense about the Second Amendment, please feel free to use the following text as a stock response:
The rights in the Bill of Rights are not absolute. You have free speech, but you can be sued for slander or libel. You have freedom of religion, but the law prohibits polygamy. The same limitation is true for the Second Amendment.
For the first couple of centuries of the Republic, constitutional law held that the Second Amendment's right to bear arms provided for a militia, and we have one, the National Guard. In 2008, the Supreme Court ruled, 5-4, in District of Columbia v. Heller that the Second Amendment protects the right of an individual to own firearms apart from service in the militia. (Please note this if you object to "judge-made law.")
But the court also held that the right is not unlimited and the government may regulate firearms. Justice Antonin Scalia's majority opinion explicitly states that the right to individual ownership is "not a right to keep and carry any weapon whatsoever in any manner whatsoever and for whatever purpose" and that governmental limitations are "fairly supported by the historical tradition of prohibiting the carrying of 'dangerous and unusual weapons.' "
And an optional addition, for snark:
People who think that the Second Amendment was written to enable the citizenry to violently resist the government of the United States — for example, if Barack Obama came around to take their guns and make them have health insurance — are people who watched Red Dawn too many times.