The caliber type "Multi" should only be used for frames/receivers that do not have a barrel attached. As soon as a barrel is attached, you must use the specific barrel caliber when making A&D entries (e.g. 9MM, 12GA, 5.56 NATO). If the firearm has multiple barrels then you would need to record all barrel calibers (e.g. 40S&W/9MM).
For more information consult the FFLGuard Help Desk