.243 AI doesn't fit into and AR-15, but it would fit in an AR-10.

I have enough big guns, and the AR platform is inherantly accurate. Of 4 AR uppers, all shoot sub 0.5" at 100 yards, and two uppers I assembled myself for $395 and $500 respectively. The others were assembled by custom shops and cost $700 and $1200. Funny how mine shoot just as well as theirs.

The guy who built my .458 upper also has a line of .308 Win based cartridges called the "Rhinos". He does whispers and other neat things, but I want an upper I can assemple myself.