The constitution, and more specifically the 2nd Amendment, does not give the people the ability to legally bear arms. It does however state that the people have the right to arm a well regulated militia, such as the National Guard. The people's current ability to bear arms is not a Constitutional right, but a government granted privilege that can be restricted or taken away.
Many people arm themselves in order to protect themselves from government oppression. This method of defense is outdated as it has become for more effective and internationally accepted to oppose the government through civil disobedience and peaceful protest. This has been proven highly effective with examples such as Gandhi's overthrow of the British Government, and the Egyptians recent overthrow of Mubarak.