[MRM]

I know for certain that an otherwise legal gun (i.e., not crossing California with a gun legal in Nevada but not California) can be transported across the 50 states as long as it is unloaded, cased, and not transported in the passenger compartment.

Many states have less restrictive transportation laws, but no state is more restrictive than unloaded, cased, and not accesible in the passenger compartment. If you follow this common sense rule, you'll be OK.