Just by the by, I am about to hand over my Wm. Powell side hammer top lever rook rifle to Holts for their next auction. It was originally a 100 bore, and then bored out to .410 2.5" between the wars. The octagonal barrel was turned down very carefully to make it a proper shotgun, with half choke.
I bought it 20 years ago and asked the late Derek Fearn to make me a new rook rifle barrel for it as well. It is octagonal, and he hand-filed the complete barrel from a match 7.62 barrel. It is now chambered for .32-20 and is very accurate, but I no longer compete in rook rifle events...
HB