Only curious, have you come up with a rear sight idea for hunting purposes. It truly looks set up as a long range match rifle. Very nice rifle, congrats. I'd think slug the bore, take a chamber cast, and decide what to do from there.
I'm not thinking that is the rifle in question.
His rifle must be a very early style action with an action number that low.
Pictures from the OP would clear it all up.
I can maybe help you out a bit, having a 461 Gibbs myself.
Your rifle, having Metford rifling, will probably have a .461" bore and grooves almost .469-.470".
Most Gibbs 461s are tight chambered, meaning that the biggest bullet that fits a fired case will be at bore size. This makes working up a NfB load very challenging but full black and duplex loads do the trick.
Paper patched bullets can be made by sizing back .458s to around .453 and patching up to .461" if indeed you have such a chamber. That at least will get you started shooting without laying out for a new mold.
Brass can be bought from Bertram in Australia but can also be made from 500/450 No.1 or if headspaced off of the shoulder, any 500, 500/450 or 470 Nitro Express brass.
Looking forward to seeing your rifle!
