I think a 6.5x55 improved would be a winner using Lapua brass, performance knocking on the heels of the 284 or '06 case, very efficient and decent bore life. I think it's called BJ Arch.
Of course PO talked of a 6.5x57 AI, 260 AAR....
Rem might have sold more 6.5-06 than 260 chose one of A-Square's rounds. Not sure if it would fit - but wonder if a 6.5-06 would run into a 25/06 chamber, and cause liability concerns.
JB - in a higher capacity round like the 06 or larger, an 8.5 - 9 twist as you know spins a little faster so given one might see a little pressure reduction, not a bad thing in these larger small bore rounds, an 8.5 - 9 might be preferred by many. I suppose they may foul a tad less as well, than a similar 8t.
No doubt a 9t can be borderline in some 6.5 rounds depending on bullet/mv, etc. It seems to be more spotty in smaller rounds, not so much in the larger ones.
