As per C.R.S. 12-26-101, “Firearms” means a pistol, revolver, or other weapon of any description, loaded or unloaded, from which any shot, bullet, or other missile can be discharged, the length of the barrel of which, not including any revolving, detachable, or magazine breech, does not exceed twelve inches.”

This definition however does not include firearms, for which ammunition is not sold or which there is reasonable ground for believing are not capable of being effectually used.[i]

All individuals, firms, or corporations in the state of Colorado engaged in the retail sale, rental, or exchange of firearms, pistols, or revolvers shall keep a record of each such transaction.[ii]  The record shall be made at the time of the transaction in a book kept for that purpose and shall include the name of the person to whom the pistol or revolver is sold or rented or with whom exchanged; his age, occupation, residence, and, if residing in a city, the street and number therein where he resides; the make, caliber, and finish of said pistol or revolver, together with its number and serial letter, if any; the date of the sale, rental, or exchange of said pistol or revolver; and the name of the employee or other person making such sale, rental, or exchange. The record book shall be open at all times to the inspection of any duly authorized police officer.

A resident of Colorado can lawfully purchase or receive delivery of a rifle or shotgun in a state contiguous to the state of Colorado, subject to the following restrictions and requirements[iii]:
(a) The sale must fully comply with the legal conditions of sale in both such contiguous states;
(b) The purchaser and the licensee must have complied, prior to the sale or delivery for sale of the rifle or shotgun, with all of the requirements of section 922 (c) of the federal “Gun Control Act of 1968”, applicable to interstate transactions other than at the licensee’s business premises.

[i] C.R.S. 12-26-101 (b)

[ii] C.R.S. 12-26-102

[iii] C.R.S. 12-27-102