I’ve tried, but I can’t warm to the .243. It’s billed as having three great attributes: it’s death on varmints, it’s adequate for

deer-sized big game and it is a great cartridge for young hunters because of its moderate recoil. I think it is a poor second-choice-if that-for any of these tasks.

More here.

I must say that I agree with the following comment by Ben on the blog post

Well, if everyone had money to dedicate to a varmint rig and a deer rig then yes, going with a .22 and a .25 or 7mm would make perfect sense. However, since this isn’t always affordable some people split the difference and find that the .243 fits the bill. 55-70grns for varmint, and 85-100grns for deer.

As for the .243 being too big for varmints – a .222 is too big for most varmints, realisticly a .22lr is all you need to kill most varmints – people just like the high powered .22’s because they reach out considerably farther – so too does the .243. And if you’re looking to save coyote pelts it can expend energy in a hurry, unlike the heavier .25’s.

At the end of the day though, if you’re hunting armored deer or Wile E. Coyote then yes, by all means you certainly need more than the little 6mm.