I usually don't dive into hose kinds of discussions. But it depends alot on who's behind the rifle. I have a buddy with endless patience, who's a stellar shot, and would watch game walk away if not a perfect broadside shot. He would do fine with a .223.
If given 1 chance on a bull in a week, which shots would you take?
I really don't think it takes a magnum. I KNOW a 7x57, 7mm-08, 308, 30-06 with partitions or accubonds will give enough pentration to break a shoulder or angle thru a lot of tissue to reach the vitals if I do my part on 99% of all shot opportunities. The Europeans have great luck with 6.5x55s, but I've never used them. Honestly any smaller than that and I would be very careful with shots and understand it may cost opportunities.
If I were setting a rifle up for a child or my wife, it would be a 7x57 or 7mm-08. Easy to shoot, but good enough penetration for most situations.