I've hunted deer here in western Oregon with the .243, but never Elk. But I also own rifles in .270 Winchester and .30-06 that are available to me. So I can easily use a rifle of larger caliber that I am familiar with.
What I can give you some feedback on, though, is that I think that it is indeed a good idea for your friend to stick to using his own rifle that he is familiar with, instead of something brand new, or borrowed from someone else. It is easy for even the most experienced hunters to get "buck fever" at the crucial point of a hunt, when their quarry is in sight. So you want to be using a gun that you are totally comfortable with, and experienced in using.
I honestly think that few people do hunt elk with the .243, which probably accounts for the lack of responses to your post here. However, if you are a member of the Oregon Hunters Association, the photos section of their members magazine is regularly filled with hunters in western Oregon that have taken deer with the .243. You especially see all kinds of kids and women taking deer with the .243. But you also see quite a few men using them too. Hunting in the western part of our state is generally a shorter range scenario than in eastern Oregon. And out to 200 yards, the .243 does pack quite a good wallop.
If I somehow found myself only able to take my Remington 700 .243 on an Elk hunt in western Oregon, I would simply do two things:
First, use a full power load with a tough bullet. Something like Federal's 100 gr Nosler Partition load or Hornady's Light Magnum .243 load. And make sure that it is perfectly sighted in on my rifle.
Secondly, be selective in your shots. Pass on any shot where you are unable to get into a position where you are not fully confident on placing your bullet well on the animal. And keep your shots to under 200 yards. Again, most terrain here in Western Oregon is typically not going to offer a lot of long range shots, as it does in the eastern part of our state.
The most important assets to any hunter is their own skill level, and judgment. That is far more important to whether they will be successful than what caliber firearm they carry.
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