If you want more info please feel free to browse through the Reciprocity Thread.
In a nutshell a State for example cannot levy different property taxes on residents and non residents solely based upon residency.
Another example is Md CCW Permit....it treats everyone like shit regardless of residency, thats perfectly constitutional.
Its not that a state must recognize your permit from another state, rather the state cannot pass laws that treat citizens and non citizens unequally (Generally speaking).
What does this mean for the current gun laws in NY. For example you are required to have a handgun license in order to possess a handgun in the state. In most areas (not NYC) they are not terribly difficult to get. But they are not issued to non-residents. As a non-resident, short of a few exceptions for sanctioned events you are absolutely not allowed to legally possess a handgun within the state.
Isn't that unequal treatment of residents vs non-residents?