Not clear on the muzzle thread for the use with a 6.5 but with use of .30 mag calibers the minimum dimensions for a 5/8x24 is .725 barrel. At 26 inches a #4 Bartlein is .750 so 5/8x24 should work. The issue with mag calibers with smaller dimensions is belling. using a #3 most I have spoken with use a 9/16x24. I'm sure there are people on this site that are more familiar with the muzzle dimensions minimum appropriate for barrels . The Bartlein 3,3b,and the 4 all have the same breach dimensions the dimensions change at 6 inches forward.