Well I guess I must disagree.
Just today I shot 3 different double rifles, a 470 Chapuis, a 470 William Evans, My 450 No2 and a 500 Jeffery Bolt rifle.
All rifles were shot by me off hand at 25 yards and I also shot the Evans sitting. All the rifles would shoot 2 shots almost touching at 25 yards. As I shot these rounds from "a field position" I "could" expect "similar" results in the field in Africa.
One of the other shooters shot the Evans double, he was thinking about buying it, off the bags at 25 yards.
The rifle did not shoot very tight groups for him from the bags. He thought the rifle did not shoot good. After I shot a few rounds off hand and 2 sitting, he realized the rifle shot fine and that he needed more practice.
I think that if more big bore shooters would give up the bench and shoot from field positions, they would do much better when hunting.

When hunting Dangerous Game you will not find any "stinking" benches.
Get close, shoot off hand, use the Force.