Double rifle! Shorter.

As for velocities I don't have a .416 so I just use the armchair expert knowledge method of regurgitation!

Real user actual muzzle velocity testing is usually more reliable.

Barrel length? I think a large capacity cartridge with a short barrel is a Billy silly. The cartridge should burn it's powder before the bullet exits the barrel. Use a smaller cartridge for a short barrel. Probably little loss of performance.

I think a carbine length barrel (20 inch) with a .458 Win Mag makes more sense than most .416s.