Re: 308 brass necked down to 243?
You can certainly neck the 7.62 brass down to 243, and have done quite a bit of this myself. While in the army, I had a virtually unlimited supply of LC Match brass, and a very limited budget, so this was what I fed my own 243 back then. There are some things to watch out for, however. One is neck thickness and clearance of the loaded rounds within the chamber. You will likely need to either inside ream or outside neck turn the reformed cases to ensure that you have sufficient clearance. The other consideration is the case thickness, and the need to revise the loads over what you'll seee printed in manuals. Military 30-06 and 7.62mm brass is generally thicker than their civilian counterparts, so you'll need to reduce the loads a bit below what the manuals will recommend. Oddly enough, this isn't the case with 5.56mm/223 Remington brass, and military stuff is virtually no different than most civilian makes, at least in terms of capacity.
This is a pretty severe necking down and you'll be moving quite a bit of metal in the process. Some degree of work hardening is inevitable here, so I'd also recommend annealing as a final step (at least), or possibly an intermediate step if you're using an intermediate die, such as a 7-08, 260, etc..
That said, I did this due to some pretty severe financial times back then (military pay being what it was/is), and was pinching pennies pretty hard. Today, I still reform cases for wildcats and some otherwise impossible to obtain obsolete cartridges, but that's about all. It's a pain, and there's too many good commercial 243 cases available on the market to mess with this particular conversion, at least in my opinion.
Hope this helps,