Your just old fashioned lol
The way I see it the .243 bullet has been engineered perfectly for its job of hitting a direct spot without too much variation in height and very quickly, if you are unable to hit the killing area of a deer at the distances a lot of them are shot at with a scope and bipod/ sticks etc, then you shouldn't be taking the shot and if you can do that then a .243 does the job perfectly and also allows it to then become a bit of a utility rifle at the same time.
I personally don't see the point in setting up 2 good rifles at £2000 per setup each when I only need one.
I'm half tempted to hold off on a .223 and exchange the slot for a .243 just because the market is full of them at great prices for a well setup one and it can do the job on fox and deer, the only time I would look into say a .308 would be if the say a place had grain restrictions on a shoot I wanted to book.
I agree though that without a scope etc a larger round should be used to ensure a humane kill as it makes up for the lack of marksmen ship
I am old fashioned in a way. I like hunting to be a challenge, find fieldcraft to be the most enjoyable part and am happy coming away with nothing. So I choose old fashioned kit as it increases that challenge due to open sights, poor trajectory at longer range and, in my case, one shot.
Having said that I don't agree with all of your points. I do agree that the .243 does its job nicely and although maybe not ideal for everything it is a very good compromise. I also agree, as I am sure everyone on this forum does, that you should be able to guarantee a hit in the vitals at whatever range you are shooting at.
Quick clarification. The .243 bullet isnt in question here, the .243 Winchester cartridge is. All sorts of .243 bullets exist for all sorts of reasons but none of them do anything without a cartridge behind it.
Quick objection. A large round doesn't make up for bad marksmanship. More power can SOMETIMES allow for slightly off placement. The bullet has to hit a vital area. Doesn't matter if its a .243 or my .458, if it doesn't hit in the vitals it won't be a humane kill.
All other things equal, all the extra power means is that if you are slightly off and hit the heart but not directly is that is could do more damage than a less powerful round. It also means that bones cause less of an obstruction.
But there are so many variables (weight, velocity, S.D, construction, quarry etc) that this is nowhere near true 100% of the time.
I don't feel that any increase in bore size (over the .243 minimum) is a substitute for good shooting. The optics you use or don't use are irrelevant, it's all about hitting the vitals. If Mr.A can do it at 200 max with a scope and Mr.B can do it at 100 yards max with open sights then that is their respective range limitations. It doesn't mean that Mr.B should get a bigger rifle and push those limits.
Grain restrictions?!?!?!? I better stock up on bread
