I've used the Sierra 63gr "semi spitzers" in my Win model 70 varmint. It has the standard 1-14" twist.
Shooting varmint bullets and with me having a good day, it does 3/4" groups. Shooting the heavy "deer' bullet groups were never better than 1.5-2 inches.
Which is entirely adequate accuracy for deer at most hunting distances.
The smaller the bullet, the more surgically precise the shot needs to be. If you're that good (and some people are) use anything you want. But, if you only
think you're that good, and you're not, the odds of wounded and possibly lost game increase hugely.
The 243 is a great combo deer/varmint cartridge, but is borderline for anything bigger than deer.
I'd agree this is the general wisdom, but will allow that border is defined by the skill of the shooter, more than the caliber being shot. I've seen guys on both sides of that borderline.
I met an old boy who had just taken his sixth (6th) elk with his .243 Win. He though it was the perfect elk rifle, and for him, it was. He hunted in deep timber, none of his shots was more than 70yds, most less, he only took head or neck shots, and was the kind of guy who could and would pass up any shot where he wasn't certain of putting the bullet exactly where he wanted. The rifle was light, didn't kick, and he had taken half a dozen elk with it, and still had over half the box of shells he bought when he got the rifle.
And on the other side of the coin, I have seen guys shooting .300 Megamagnums (and bigger!
) who would be lucky to hit an elk anywhere, because they couldn't shoot their rifle worth a damn, it kicked the snot out of them and scared them (flinch). But they all seem to
think they can drop one at 600yds, because of paper ballistics (or they read it on the Internet...
)
The rifle matters, but not as much as the person pulling the trigger.