6.5x47L brass has been typically, wonderful; coupled with a small rifle primer has great potential for and efficient, accurate round. I have read several places where folks claim 243 Winchester velocities. Powder capacity is 6 or 7 grains less. Just not going to happen.
Dennis, the 6x47 I recently sold averaged 2956, 105 Amax over RL26, 22.75" barrel.
I checked my notes and my 243AI that I recently sold, averaged 3018, 105 Amax over RL22, 22.5" barrel.
My 243 donor for the AI above averaged 3005, 105 Amax over RL26, 24" barrel.
Out of a 22" barrel I believe it's a tie.