What a HUGE can of worms you opened on this one!
The 6 PPC has won a ton of SR matches. The 6 BR has won alot of 600 yd benchrest matches, but they're attempting to upsize it just a bit for the 1000 yd game. From 6 Dashers and Long Dashers, to 6 BRX to 6X47 Lapua and AI 6X47 Lapua, 6 XC is David Tubbs creation I believe and alot of guys are trying it at 1000. The list goes on and on and on and on.........and I am only remembering a very few at the moment.
Oh, lets go even bigger and there's the 243, 243AI, 6MM Rem. 6mm AI, 6-284, 6-06, 6 WSM.........I am probably forgetting dozens in the mix.
6mm SM, 6mm CM, 243 WSSM, 6mm RSAUM, 6-250 AI, ............?? TONS of em.
I personally know that a 6mm Remington will last a very very long time. Lost count of the number I shot at PD's before it was shot out, but it's not the
most accurate 6mm on the line. Gotta go smaller case capacity if you want ultimate precision. Sacrifice a little velocity to get better accuracy and even longer barrel life
FWIW, I am playing with the AI 6X47 Lapua as of most recent. Not saying it's the best, but it's gotta be right in the ballpark. 6XC is just slightly bigger, 6 Dashers are a touch smaller. Seems I read somewhere that the 6 LD has a world record at 1000 yds, but I am going off memory which tends to get distorted sometimes.