The current 10 shot group world record @ 1000 is held by a 6.5mm (.264"), and 1/2 MOA groups with 6mm and 6.5mm will get you in the running, but won't guaranty a win by any means.
After decades of running with the .30 cal dogs on the 1,000 yard range in .308 and .300WM I was ready for something different.
I was impressed by what I read about the 6mm and 6.5mm and decided to give .243 a whirl. Pleased with the results, I punched it out to .243AI and I'm really happy with the results so far.
Next up, I think I'm going to build a 26 Nosler. It's basically an unbelted .264 Ultra Mag.