I recently bought a MK IV 5.5"target model. Thinking of adding several uppers, 7"or 10" hunters. And maybe a Volquartsen tactical with a 3" barrel for a suppressor host. I did not realize each upper must be registered as a pistol with accompanying FFL fees and checks. All my other pistols have serial numbers on the frame. I can buy any upper for my AR pistol lower with no problem as again the serial number is on the lower. Just wished Ruger would've put the serial number on the lower frame instead of the upper. OK rant over.