Gentlemen, my very non-original comment is....
It's the bullet that does the work. Not the cartridge, not the kinetic energy, but the bullet.
Handloading resources for that 'anemic' cartridge on the interweb indicate that good hunting bullets that will properly upset and expand are available in that diameter or can be sized to work.
This is a double rifle, so presumably you have the fastest possible second shot available. Double rifles are traditionally employed at short ranges.
A caliber with the energy of a .357 should be entirely adequate for deer at 50 yards when loaded with a good bullet.
Pal of mine did just that with a 4" barrel S&W a few years back. The guy is a well practiced pistol shot. He double actioned the second shot immediately after the first. They landed within inches of each other. Dead deer.
This rifle would be tons of fun to load for and hunt with.
