Ditto to all those explaining the 6 Creedmoor does in factory form what the 243win can only do with custom barrels and custom reamer prints.
In fast twist 6mm’s, I used to shoot a lot of 105 A-max’s And 105 VLD’s. Today, I shoot 103 and 108 ELD’s and 105 Hybrids. I won some 110 A-tips yesterday, looking forward to trying them out.
I grew up with Magnumitis, starting my deer hunting and coyote hunting life ~30 years ago with a borrowed 270win, after which I bought my own 30-06. I bought a 243win as a dedicated coyote and prairie dog rifle about 20yrs ago, and quickly realized how efficient and effective it was for deer sized game. The 243win became what I consider to be the most efficient whitetail hunting cartridge on the market, with no concessions. But I always wanted them to do more - so I blew out the shoulders to an AI, ordered custom barrels, dies, and reamers for shooting proper weight bullets for the case capacity... all spending a lot of money to do what can be done today with factory rifles and ammunition in 6 creedmoor.
More reading for
@Roamin_Wade:
6mm BR database article on 243win.
In 25-06, the 131 Ace from Blackjack and blowing the shoulder out to AI is a real game changer. Not quite a 25 RSAUM, but really quite capable.