How does 2nd amendment limits the federal government.

The limits Federal government they have is that it prevents the federal government from banning private ownership of firearms by legal, law-abiding citizens. In so doing, it provides a balance against the power of the central government to seize absolute control by force of arms. It also provides for the citizens to act in self defense of person and home against criminal actions by outlaws. It also provides a pool of armed citizens to act in defense of the nation and state (not so much in this day, but when there was little or no standing army, this was important).