I have found the mid .40cal is neither fowl or fish. The .416Rigby is a good cartridge but...its not a stopping class caliber here one would need more bulletweight and caliber. It´s not an ideal caliber for plainsgame either. One doesn´t need that much power to bring down animals up to eland size. The .416Rigby has longrange potential because of its case design...but big game are not shot at long range..it´s shot at close range where I see a heavy double is better. For long range plains game shooting a .338win, 270wea etc is better choice. Yes the .416Rigby can max it´s longrange potential with a 300grain spitzer bullet loaded up to .416Wea performance...but the 300Win mag will do the same on any antilope....and with a lighter rifle. The .416Rigby is versatile because bulletweights from 250-450grain is offered. However if I had to shoot 250grain bullets I wouldn`t use the .416Rigby I would use the .338...and if I had to shoot 450grain bullets I would go for bigger caliber because then it would be needed.
If I were today to have a mid .40cal rifle a .404Jeffery would become my choice. It can be made on a stand length Mauser action and build lighter.