Two problems here.
First OCDO is primarily concerned with OC of normal handguns, properly holstered. Lengthy discussion of long guns is discouraged - there are other sites for that.
Second is that rules/ordinances on long guns are frequently controlled by hunting laws including but not limited to state laws and agency rules - local laws play a part too. Preemption is not total.
Generally, it is best to transport long guns unloaded and if in the passenger compartment, very visible i.e. NOT behind the seat in a PU.
In a gun rack (open), cased or in the trunk are considered OK. Loaded and accessible may be prima facia evidence of hunting.