Definately the 8" twist will be better. Unfortunately, the M-48 is made with the 1:9" twist. Perhaps, someone with a .260R w/1:9" can chime in. Or, better yet someone from Nosler. :grin:
260 rem is designed to shoot lighter bullets. 100 - 140.
I have 6.5x55 1:8 twist and i shoots 140 very very well but does not as much 100 grainers.
I talked to Bartlein Barrels and they recommended 1:9 twist if I don't plan to shoot heavier bullets than 140 gr. Lapua makes brass for it right now.
I hope this help.