Q. Are Colorado residents required to register their firearms?
A. State law prohibits firearms registration by local or state government (the federal Brady Act is another matter, though that's not "claimed" to be registration). As is often the case, there is a catch. Colorado has an unusual state constitution, which gives some cities co-equal power to legislate over certain items, with the State Legislature. These are so-called home rule cities. Denver is a home rule city, and sued over this firearms pre-emption effort the legislature passed. In the case City & County of Denver v. State of Colorado, 03-CV-3809, the Denver District Court partially excluded some Denver ordinances from being pre-empted. http://www.courts.state.co.us/Media/Opinion_Docs/03cv3809order.pdf. The decision was appealed to the Colorado Supreme Court, which upheld the trial court decision because it split 3 to 3 on whether to affirm or reverse the decision. See http://www.courts.state.co.us/Courts/Supreme_Court/opinions/2004/04SA396and05SA22.pdf
As a result Denver still maintains a number of gun control ordinances, including requiring a registry of all firearms purchased in Denver. Denver does not require registration of firearms possessed in the city. Denver had required that a copy of the registry for purchases in the city be sent to the city, but repealed that ordinance. See R.M.C. section 38-123 for the purchase registration requirement.
C.R.S. Section 29-11.7-103. Regulation - type of firearm - prohibited
A local government may not enact an ordinance, regulation, or other law that prohibits the sale, purchase, or possession of a firearm that a person may lawfully sell, purchase, or possess under state or federal law. Any such ordinance, regulation, or other law enacted by a local government prior to March 18, 2003, is void and unenforceable.
C.R.S. Section 29-11.7-102. Firearms database - prohibited
(1) A local government, including a law enforcement agency, shall not maintain a list or other form of record or database of:
(a) Persons who purchase or exchange firearms or who leave firearms for repair or sale on consignment;
(b) Persons who transfer firearms, unless the persons are federally licensed firearms dealers;
(c) The descriptions, including serial numbers, of firearms purchased, transferred, exchanged, or left for repair or sale on consignment.
Denver Revised Municipal Code Sec. 38-123. Identification and records concerning sales, etc.
(a) Every person who sells, rents or exchanges at retail any weapon designated in sections 38-117 or 38-122 shall require the vendee, lessee or person with whom such exchange is made to furnish more than one (1) type of identification before such sale, rental or exchange is consummated.
(b) Every person engaged in the sale, rental or exchange at retail of any weapon designated in sections 38-117 or 38-122 shall keep a record of each such weapon so sold, rented or exchanged at retail and of the vendee or lessee thereof, or person with whom such exchange is made; said record shall at all times be open to inspection by the manager of safety or a duly authorized agent, or by any police officer of the city and shall be made at the time of each such transaction in a book kept for that purpose, and shall include:
(1) The name of the person to whom such weapon is sold or rented, or with whom it is exchanged;
(2) The age of such person;
(3) The occupation of such person;
(4) The residence and, if residing in a city, the street and street number of the residence of such person;
(5) The make, caliber if any, and finish of the weapon so sold, rented or exchanged, and the number or serial number thereof, if any;
(6) The date of the sale, rental or exchange of such weapon;
(7) The name of the employee or other person making such sale, rental or exchange of such weapon; and
(8) A description of each of the types of identification furnished by the vendee, lessee or person with whom such exchange is made, together with any serial numbers or other distinctive features noted in or appearing on each such type of identification so furnished.
(c) It shall be unlawful for any person who purchases, rents or exchanges any weapon designated in sections 38-117 or 38-122 to give a false or fictitious name, or any other false, fraudulent or incorrect information to the person from whom such weapon is purchased, or rented, or with whom such weapon is exchanged at the time such purchase, rental or exchange is made.