There was a Merkel double in .416 Rigby available locally. It was a sweet handling gun, fortunately I resisted temptation... I think it’s about $10 a bang for the ammo.
The 416 has great power. Part of my preference in a charge stopper is the cartridge taper, which is why the .375 is the best choice. The rounded shoulder .458 and straight sided .416 does not feed very dependably for a follow up shot, which is most always usually the case in a frontal charge.
Of course, a .223 can kill a bear with the right angle and shot placement. But in a frontal charge, there are few options. Heavy caliber, high energy is the only choice to be safe. (Not to mention the most dependable rifle design, usually in a Mauser action.)