I think words like adequate,humane,and ethical, all have to do with the ability of the hunter to make an accurate shot, more than they do with caliber size.
I hunt with 30 caliber myself, but know several hunters that hunt with smaller and larger calibers, that all have one thing in common. They can all shoot 1/2" groups with their rifles at 100 yards, all day long.
When they have hunted with me, almost every thing they shoot is DRT, due to the placement of the shot.
I have had to put down cattle and horses with a .22 LR, and there is no caliber out there that will dispose of them any faster, so I would imagine about any legal caliber would work for game. The key is shooting the rifle until the accuracy is achieved by the hunter to make that adequate,humane,and ethical shot.
Just my 2 cents
