Knock Down
Look guys, you can anything with anything if you make a perfect shot.
The trick is to provide a little extra margin in case: the animal moves at the last second, the wind moves the round a smidge, the animal is larger than you thought or a hundred other problems that can make for less than the perfect shot.
The .243 is a great round. It is a good antelope or varmit round. However, in my opinion, it is marginal on medium to large deer. Yes, you have killed many deer with it. How big were they? How far did they go? How many had to be shot twice?
A flat shooting bullet is no substitute for shock. Larger bullets generally deliver more shock (knock down power). Therefore, if one of the above things unexpectedly happen, the margin it provides is more likely to drop the animal humanely and save tracking or a disappointing loss.
In an extreme example: I have never lost an animal to my 12 gauge saboted 50 caliber bullets. Generally, they drop immediately. On the other extreme: arrows provide minimum shock. Tracking is normal.
With the bullet selection in a 30-06, .308 or the unmentioned .280, one rifle can handle all the game.
As someone once said: Beware of the man with one rifle, he probably knows how to use it. (In spite of that, I have a bunch)