I played around this last year with setting up the “softest flattest” rifle I could. The conclusion was an sjc comp, vltor a5 buffet system(with 00 buffer), adjustable gas block, and an aluminum carrier.
My rifle has a rifle length gas system and an 18” barrel.
I tried 5 different comps ,3 different carriers, 4 different buffers. After mix matching and shooting the above proved to be the best setup for softness and flatness the only downside is it seems to be a little on the heavy side.