Re: .243 vs .308
As Steve123 said I feel that to get the most out of the .243 Win cartridge you need to have an 1 in 8" twist barrel or faster. That will allow you to shoot the heavier 105, and 107 grain bullets that retain their energy and therefore show less wind drift. After all the range is often easier to determine than the wind in most situations.
A .243 shooting a 105 or 107 grain match bullet is very similar to the .300 Winchester Magnum shooting a 190 grain match bullet- with regard to wind drift and drop. It is NOT similar with respect to recoil and terminal energy. The .243 has much less of each. HOWEVER- most people don't realize that a .243 Win (or 6XC- or 6x47L) has about the same energy as a .308 shooting 175 grain bullets at 1000 yards. This is because these long 6mm bullets retain their energy better.
That being said I don't think most of the over the counter .243 rifles have a twist rate fast enough to stabilize the 105 grain or 107 grain bullets. Perhaps nowadays they do- I don't know for sure. I think most of the factory .243 Win barrels will stabilize a 100 grain hunting bullet though. The throat is also an important consideration if you want to shoot these long bullets.
All of these things pretty much add up to installing a custom barrel. In my mind if you are going to go this far then you should go "whole hog" (can you tell I am originally from Indiana?) and have a custom rifle built for 6mm XC (David Tubbs cartridge).
Why do I say this? Well although I agree with Steve- I think he is being rather optimistic thinking that someone can get 2500 rounds from a .243 Win, but I could be wrong. I would guess around 1800 rounds for .243 Win, 2500 for 6mmXC, and 3500 for .308. The 6XC case achieves within 50 fps (negligible) of the velocity of a .243 while using around 5 grains less powder.
Also the 6XC has a sharper shoulder than the .243 Win. Why is this important? Well with a sharper shoulder when the burning powder is being blasted forward it bounces on the inside of the shoulder and then impacts the inside of the case neck (mostly). This is much preferred over the powder bouncing against the inside of your barrel throat- as happens with a low angle case shoulder. This burning powder is what creates the cracks and erosion of your throat. You can only move your bullets closer to the lands so many times before its time to re-barrel or at least set your barrel back.
The downside to 6mmXC or other cartridges in this class (6.5 or 6x47L) is that it is going to be hard to find brass at your local gun shop.
The question is - are you really going to wear out a factory barrel? I doubt that I could, as it probably would not interest me much due to the fact that it wouldn't be real accurate.
Now you understand why my name is "6mmFan".
Dirk
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