......hunting moose and elk with the .243 would be downright stupid--even if it somehow worked out ok.
That's an opinion, and maybe the right one, for you. OR for me. But not for everyone. I won't recommend it, personally, I think the .243 is too light for game that large, but it can, and has been both successfully and humanely used.
Some years back I met an old boy in his late 70s, who had just taken his 6th elk with his .243, He considered it the perfect elk gun. Light, handy, almost no recoil, accurate, etc.
Now, he also didn't take shots more than 100yds, most under half that, and only took neck/head shots. And he was the kind of guy would not take a less than perfect shot. For him, it worked very well. Sort of like the .410 shotgun, within its limitations, and in the hands of an expert, it works very well.
The further you get outside those things, the less effective & efficient it is.