Which of these firearms requires written permission from the Registrar if altered?

The requirement for written permission from the Registrar when altering a firearm primarily applies to fully automatic firearms. Fully automatic firearms are heavily regulated due to their ability to fire multiple rounds with a single trigger pull, which can present significant safety concerns. Alterations to these types of firearms can affect their functionality, performance, and compliance with legal standards. As such, it is crucial that any changes to fully automatic firearms receive oversight and approval from regulatory authorities to ensure that they continue to adhere to safety and legal guidelines.

The other categories of firearms mentioned may have different regulations regarding alterations, but they do not typically necessitate written permission from the Registrar in the same manner as fully automatic firearms.