Lagopus,
Your smooth bored .300 was, I suspect, smooth bored at a time when original Rook rifles were still “on ticket” and deemed to require a Firearms Certificate but by being smooth bored could be held much more easily on a Shotgun Certificate.
As .300 (or .295) Rook is on the Obsolete Calibre list you can clearly hold it as s.58 antique (assuming you do not intend to shoot it).
You are not quite right in describing in as a “calibre that does not exist” as back in the day, in the same way that you can still buy .22 rimfire shot cartridges for pest control, .300 Rook as well as .360 No. 5 etc cartridges were offered with shot loads.
I have a star crimped .300 Rook shot cartridge, it has a similar crimp to the Federal .22 Long Shot cartridge shown above.
