If I built a 6mm other than a BR or 243, I would seriously look at a 6/6.5x47 Lapua. Great brass - similar speed - stellar accuracy, reasonable bbl life, forget trimming like a 243, and it feeds as is w/o mods like a BR might need.
It's what my 'smith' shoots, and sub 2" at 600 yds gets my attention!
All you do is bump the 6.5 brass down to 6mm, load and go. Simple.
That said, brass and die cost will be more expensive, but nor forming.
A 6mm XC would also be of interest to me.
Having owned several custom 6mmBRs, and seeing how accurate they are, pleasant to shoot, and deadly thru deer to 400 yds, I am sold on the value of a ctg. that has a proven track record of accuracy as it's SOOO easy to load VERY accurate ammo for and get anywhere from .1-.5 moa....all day long w/many bullet combos, esp. assuming you use an 8" twist.
Do a little research on the above rounds if inclined.
In the end, for a sporter rifle - esp. ULA a 243 likely will shoot as good as you can, but I would defintely use an 8" twist in whatever you build. Properly built and loaded, I like all 6mm's thru 6AI, but would stop there before getting a bbl burner like a 6/284.
That's just my thoughts. Good luck.
