According to a local PSP Trooper; "there is absolutely nothing wrong with a gun rack in the back window, providing the vehicle is legal with 2 outside mirrors, and the guns the rack is carrying are unloaded".
Hope this helps.
One other caution is watch where you park with guns in a vehicle. It wouldn't take much to bust the glass and no longer have a gun in the window.
Especially since many long guns are not registered same as hand guns; it would make them easy targets. IMO.