....it has been my experience that I don't know when, where, or how an animal is going to appear or at what angle it may present itself for a shot.
No, you can't
know these things with a high degree of certainty..... but if you don't have a pretty good idea, then you have not scouted your hunting spot very well.
My best spot (a small group of trees in an old oxbow on a dry creek, where 5 trails come through ) has a lot to do with why I chose the bullet I did- many of the shots are at bucks walking nearly right at me, at short range- they come over a hill @75 yards or up out of the creek bed @ 25 yards ...... shooting them with one of these bonded or all copper/GMX bullets would make a mess of things ..... there are wide open fields on 3 sides of these trees, offering shots as far as I am comfortable shooting, as well ...... there's plenty of time to wait for a broadside shot out there ... when the animal is 25 yards away and on a mission to find that hot doe he thought he smelled, not so much..... So these spendy "controlled expansion" bullets are right out, for my application.
Light for caliber conventional cup and core bullets at Ludicrous Speed and short range can fail to penetrate : 20 years ago, I had one of the old aluminum pointed Winchester Silvertips disintegrate on contact- wound was about 3"x3"x3" on the side of the deer- deer died and was recovered, but I think he died of surprise- he still had a good lung, heart was intact, there was not a huge blood trail, and ran well over 100 yards .... had he been facing me, and the bullet had hit his sternum instead of a rib? I think of that deer when I hear folks advocating for 110gr in a .270 or 85gr in .243 pushed as fast as they can be made to go ..... my bullet was the standard weight and velocity for the .270WIN.
I went to 150 grain SGK's after that, and have found that they will come apart inside the chest cavity on deer at short range, yet hold together and exit on longer broadside shots .... shoots flat, and reasonably accurate out of my gun...... perfect.
Any reason to shoot a deer with anything bigger than a .243?
Umm.... 'cause they won't stand still and let me cut their throat?
I cant see why the 243 isnt the better choice for a scoped deer rifle for shots under 300 yards.
There have been years that if I did not take the shot that presented itself (well past 300 yards)......the freezer would have not had much venison in it ..... some years I only get 2 days to hunt, and sometimes I have small kids in tow (and so every deer in the section knows exactly where we are- maybe they think that if they are are further than 400 yards away, they are safe- there are a couple that have been dead wrong about that.
The OP did mention
Guy I know wants to buy his son a new rifle for whitetail hunting and asked me what caliber to get.
and
Keeping in mind, most hunters dont shoot as much as we do.
and
normal shooting scenarios.
If the kid in question does not shoot much, or more importantly, does not intend to shoot much ...... he'll not have any business shooting past 300 anyhow.
If he intends to get into shooting and hunting ..... maybe something bigger, stoked with reduced recoil loads for now might be the way to go.
I'd not have the confidence to try a 450 yard shot with a .243 .....without extensive load developement and testing ..... which would cost more than the gun, I think.