The original meaning of the Second Amendment was the federal government couldn't take away your right to own a gun – but whether state or local governments could was outside of the scope of the federal constitution, it was a matter for state constitutions and state laws to determine. At the time it was passed, state and local laws imposing limits on gun ownership were widespread (often on grounds of race or property ownership) – and the Second Amendment was not seen as contradicting those laws in any way.

Then they passed the 14th Amendment. And the original intention behind the 14th Amendment, was nothing to do with guns at all – it was to protect the rights of the recently freed slaves – nobody was even thinking about guns at the time they voted for it. And then, decades later, the Supreme Court started to interpret the 14th Amendment as applying parts of the Bill of Rights to the states. They used the 14th to apply the 1st to the states in the 1925 case of Gitlow v. New York. And then in the 2010 case of McDonald v. Chicago they did that to the 2nd as well. It was hard to argue not to do it with the 2nd when they'd already done it with most of the others.