Nothing happens, because there is no such offense. Colorado does not register firearms, so no one is charged with possessing an unregistered one. C.R.S. 29-11.7-102, headed “Firearms database - prohibited,” goes further: a local government, including a law enforcement agency, may not maintain a list or other record or database of people who purchase or exchange firearms, of people who transfer them unless those people are federally licensed dealers, or of the firearms themselves, serial numbers included.
That does not mean every firearm in every hand is lawful. The charges that actually get filed are these.
What Colorado Does Prohibit
Possession by a previous offender. C.R.S. 18-12-108 makes it a crime for a person with a qualifying prior conviction to possess a firearm. This is the most common firearm charge that has nothing to do with the weapon itself.
Possession of a defaced firearm. C.R.S. 18-12-103 makes it a class 1 misdemeanor to knowingly and unlawfully possess a firearm whose manufacturer’s serial number or other identifying mark has been removed, defaced, altered, or destroyed, except by normal wear and tear. People sometimes read a missing serial number as a registration problem. It is its own offense.
Carrying concealed without a permit. C.R.S. 18-12-105 covers unlawfully carrying a concealed weapon.
A private sale without a background check. C.R.S. 18-12-112(1)(a) requires a private party transferring a firearm to arrange a background check on the person receiving it and to obtain approval of the transfer, with exceptions listed in subsection (6). This is the rule people most often have in mind when they worry that a gun bought from an individual is somehow undocumented.
The Federal Registry People Are Thinking Of
One registry does exist, and it is narrow. 26 U.S.C. 5861(d) makes it unlawful “to receive or possess a firearm which is not registered to him in the National Firearms Registration and Transfer Record.” For that chapter, 26 U.S.C. 5845(a) defines “firearm” as a short barreled shotgun or rifle, a weapon made from either, any other weapon as the statute defines that term, a machinegun, a silencer, or a destructive device. Ordinary rifles, shotguns, and handguns are not in that definition and are not registered anywhere.