Going to have a new rifle built- have 29" & 31 " blanks 6.5mm 1/8.5 on hand. AMU & HV contour. I often see bbl length of 28" for this cartridge. Going to shoot (possibly f/tr)., steel LR and varmints. Does 6.5 X 284 need 28" to burn powder-must be a reason for bbl over 26"... ? 139 Lapua & 140 g Amaxes...thanx.