The magnum Magpul AC (300 Win Mag, the yellowish follower) works perfectly out of the box for 6.5x284 and 284 Win. No mods needed for either of my rifles (Defiance Deviant and a Shilen DGR, both in McMillans with PTG bottom metal). The 284 win based cartridges are a rebated rim, so they are nice and fat down at the head.
I also use the 10 round 300 win mag AICS mags with no modifications.