A display clamp, and related method, are disclosed for removably attaching a second, interchangeable barrel to a fully assembled long gun (i.e., a shotgun or rifle) for display purposes only, wherein the extra barrel's gas cylinder or lug is suspended from the long gun's magazine tube, and held in place by the shotgun's/rifle's removable magazine cap. The method and apparatus allow a retail store (or buyer) to display the firearm and extra barrel as a combination unit either vertically or horizontally oriented within display cases.