Colorado Revised Statute 33-6-125. Possession of a loaded firearm in a motor vehicleIt is unlawful for any person, except a person authorized by law or by the division, to possess or have under his control any firearm, other than a pistol or revolver, in or on any motor vehicle unless the chamber of such firearm is unloaded. Any person in possession or in control of a rifle or shotgun in a motor vehicle shall allow any peace officer, as defined in section 33-1-102 (32), who is empowered and acting under the authority granted in section 33-6-101 to enforce articles 1 to 6 of this title to inspect the chamber of any rifle or shotgun in the motor vehicle. For the purposes of this section, a "muzzle-loader" shall be considered unloaded if it is not primed, and, for such purpose, "primed" means having a percussion cap on the nipple or flint in the striker and powder in the flash pan. Any person who violates this section is guilty of a misdemeanor and, upon conviction thereof, shall be punished by a fine of fifty dollars and an assessment of fifteen license suspension points.
Note that in Colorado state laws about firearms cannot be preempted by local ordinance, although Denver doesn't seem to feel it applies to them.
I avoid Denver because they still try to confiscate vehicles stopped with loaded vehicles. The statute seems to state that a loaded magazine could be attached, but I worry a cop would cycle the action and then you do have a loaded rifle in your car even though you did not do it. They would do this in Denver.