For A&D Record keeping purposes, If the particular firearm you are working with is a frame or receiver you should always enter what is marked on the frame or receiver.
You can put "Multi" for the caliber in your A&D records if the receiver or frame is marked that way AND there is no barrel attached with a designated caliber. You can also put "None Marked" in your A&D records for the receiver or frame as "Multi" is not a caliber.
When a barrel is attached to a frame or receiver you MUST enter the caliber of the barrel. It does not matter what the frame or receiver says.
You should NEVER enter N/A, None, or NSF (No Serial Found) as the caliber.