Red,
While it is true that the .243 cannot be had in the AR-15 package, it can be built on the AR-10 that is used for the .308. RRA offers a complete upper in .243, with the caveat that the barrel is a 10-twist. This will likely limit you to bullets less than 85 grains or so with the barrel throated accordingly.

url=http://www.rockriverarms.com/index.cfm?fuseaction=category.display&category_id=529]RRA .243[/url]

Read my initial post in this thread for an upper that would be able to fling drag cheating 105 grain VLD's or A-Max's of that caliber at just shy of 3,000 fps. Looking forward to your progress reports....