Can a .40-65 High Wall (modern steel barrel) be rechambered to .405WCF? They are both .40 but I don't know if bore and groove are the same or if the chamber will clean up. Maybe the barrel could be set back and rechambered if the .40-65 chamber is too big to clean up? Any clues would be appreciated.

Getting stupid in my dotage, of course the.40-65 is a fatter case than the .405 so barrel would have to be set back even if bore and groove are OK. And what about twist?

Have now been told the .40-65 is a .408 and the .405 a .412 so I guess idea won't work.

As the 40/65 was a blackpowder round it could be loaded with smokless to much higher pressure.
Problems; Check the diameter of the firing pin, The barrel may not last long, and you will be ruining a nice old rifle.
If it were mine I would sell it and buy a Ruger. If your highwall is in any sort of shape
buy two Rugers.
Good luck!

Hawkins, have abandoned the idea.

Don't know what you're trying to achieve but a 40/65 Win. in a modern gun (like my Browning 1885 BPCR) using smokeless powder and jacketed bullets will do anything a 405 Win. can...and more. The real problem is jacketed bullet selection. .41 cal pistol bullets work ok but are light. The only jacketed bullets (that I know of) for the 40/65 are 260g JFP's. I've never tried it but I'm sure the Browning high wall with a crome moly barrel could handle very powerful smokeless/jacketed loads.