And a .22 killed a circus elephant at the turn of the 19th century - but he bled to death and was in pain as he died.
The hunters in Darkest Africa weren't built on the grand scale that they are, today, either.
As to stopping an elephant - I wouldn't be putting 'my' faith in a .45-70 - I'd want something far more efficient.
TR liked the .405 - in a Winchester Model 1895 - but used larger rounds, too.
You want two things - a humane kill, and the ability for a back-up shot, if needed - the Trapdoor doesn't allow that - but for well-placed, single-shot fire within reasonable ranges, it'll do an efficient job.
Blowing things up isn't hunting.
Scouts Out!