No problem with .243 Win here.
But maybe I can help enlighten with just a tiny story.
Many years ago my Dad and I got off late one morning, dawn was already breaking when we drove up the dirt road to the spring we wanted to hunt. We could see a pickup already parked up there, so we stopped short and waited at the stream below the spring. After 2 hours a deer walked across my position, only about 20 yards away. I killed him with a single shot across the chest. Close shot, but right through the chest and lungs. Dropped dead.
Upon inspection we immediately noticed an entry wound and exit wound in the gut area. As I started to open up to field dress, another hunter approached and said it was his deer, shot with a 243 at the spring. Sorry, I said, I killed the buck. He's mine.
He showed me the empty brass he picked up after his shot, a 243 Win case with Winchester head stamp. He shot the buck at close range, just like my shot. But his went so cleanly through the gut, the bullet didn't have a chance to open up in soft tissue at high velocity. A very close shot and such velocity it didn't open up, it penetrated a 6mm hole straight through. Buck hardly bled at all!!! A few spots of blood and nothing more. He ran into the brush and hid. Two hours later he moved downhill. To my position.
Now, we all agree that a bullet that mushrooms open and penetrates is best! If it dumps its energy and still penetrates straight through all the best!! But that hunter's bullet didn't get a chance to open up. It penetrated straight through without opening. I bet it hurt the deer, but no way a 6mm hole through the gut would kill. My shot across the chest killed.
Morale of the story: Some folks say "Marginal for deer" because they read it on the internet and never actually hunted the gun. But in truth... ANY caliber you shoot must be placed correctly or it won't kill. A .243/6mm must be placed carefully and accurately. Then it kills.
I don't think I'm wrong. I think I'm right. 243 certainly kills, just learn to hit the right spot and discipline yourself to shot placement.
Sorry for the long hunting story, I think it is good for all of us. You need to shoot well if you want a clean kill. 243 to 45/70, you still need to shoot well.
Thank you, brothers.