Elarski
On the 6.5-284, if used for 1K BR, guys want to run their rounds as fast as possible normally with little bag upset, that is why the muzzlebrake.
As for the different configurations, if you are talking about chambers you will find most shooting .290 neck with minimum SAAMI chamber, 1 1/2 lead and throated for particular bullet with 28-30" 1-8 twist barrels at 2900-3000 fps, few using 1-9 twist. About 10% percent are shooting some form of an improved case and overall same dimensions other than improved and maybe 3050-3100fps.
BH