"I think the .270 with 130gr bullets is a fine deer cartridge, but will any deer within 400 yards know the difference over a .243?"
I've seen way too many deer shot and lost with the 243 to agree with this statement. Bullets are better these days but in the time I'm talking about the corelokt bullet was very common. The 243 with todays better bullets is a site better but I still believe it belongs in the hands of a better than average shooter. Even today, most hunters will buy and hunt with the cheapest ammo they can find and don't take advantage of the better bullet offerings.
Larger bucks 180 pounds or more which our mature deer weigh typically run further when hit with a 243 and left a lot less blood trail to follow than deer hit with 25-06, 7mm-08, 308, 270, 30-06, 7mag etc. Mature boar hogs with thick shields travel farther as well than sows and smaller pigs. I had a blood dog and excellent eyesight got called out regularly. 100s of times over the years. Several friends started out with 243s and gave them up. They swore by them until they lost enough really nice bucks shot with them to realize it was more than a coincidence. Most went up to 7mm-08, 308, 25-06, and 270 and their trouble went away. Those are not what I'd call man card cartridges. And I certainly don't recommend them as a first cartridge with full power loads. My statement above referred to the attitude of the times I grew up in. Yet even today, you will find a 270 in the hands of most hunters in my area and it's because it works so well. Kids can handle a downloaded 7mm-08, 270 or 30-06 just fine and I load for a few kids right now that are using these cartridges. They are easy to shoot and the recoil isn't bad at all. The 243 has seen a resurgence here but as a depredation rifle. It works very well on the 80-115 pound average doe that we shoot with the better bullets that are made today. A lot of folks in the depredation role like them because deer will run far enough after being hit to make it out of the field. I prefer the Grendel, Creedmore, or 6.5-06 for depredation personally and kill a lot of deer (around 100) and hogs every year with these cartridges. I tend to lean more toward the Creedmore as time goes by. 95% of the deer I shoot drop in their tracks. But I shoulder shoot. A practice I started years ago after blood trailing deer through very tough terrain that were lung shot and traveled 30-70 yards.. I can honestly say that I only know 2 people that use 243 now during our actual deer season when targeting mature bucks. Both are excellent shots and shoot their deer in the neck. I have several spots (bean fields) where shots are possible out to 700 yards. That is pretty typical for my area. In other areas of the state where there's very little farming shooting opportunities are a lot shorter. Our terrain in the Lowcountry has changed dramatically in the last 30 years or so since hurricane Hugo. Very thick, overgrown clear cuts, bays, and swamps cover a lot of our terrain that used to be much more open. A deer that covers a lot of ground after a hit can be a nightmare and often is not recovered or is found later by buzzards. If you want to hunt with a 243 that's your call. Have at it. If I can do something to improve my odds of anchoring a deer. I'll doggone sure take advantage of it. And I absolutely hate recoil and muzzle blast. As for youth, we start them off with a 6.5 Grendel or Creedmore. Or downloaded 308, 7mm-08, 270 or the like with a lighter bullet at 2600-2700fps. On paper the Grendel may not look like much but it has proven time and time again to be a very efficient killer. Surprisingly so. And we get good blood trails with it. It is my bottom line cartridge. I don't go smaller.