AGS,

One reason for the number of conversions was an unintended consequence of the Firearms Act. Prior to the introduction of the Obsolete Calibres List both original calibre Rook rifles and those tubed to .22 were required to be held on a Firearm Certificate with the need to demonstrate “good reason” to possess and use (difficult when the ammunition ceased to be made).

It was much easier to re-bore them to .410 so the owner just needed a Gun Licence (10 shillings over the counter at the Post Office) or after 1968 a Shotgun Certificate with no good reason having to be demonstrated.