[AndyTheBear]

In other words, the courts have held that:

Congress shall not...

Means that all levels of government shall not, howerver:

...shall not be infringed.

Means shall not be infringed by Congress, but may be infringed by all other levels of government (especially the judicial).

[robertpaulsen]

Prior to the 14th amendment, the courts held that the Bill of Rights protected those rights from federal infringement only. The states were free to infringe on those rights (and did) unless their state constitution protected them.

Subsequent to the 14th amendment, some (not all) of the rights contained in the BOR were protected from state infringement also. The second amendment isn't one of them.