I have had the best luck with flat-based bullets... all of the boat-tails that I tried tumbled. I use Clays.
Any boat tail greater than an 11 degree boat tail will cause tumbling (i.e. stability) issues. The worst one's are the 168 SMK and 168 A-max. They tumble for the same reason as a subsonic as they do when they transition from supersonic flight. They cause turbulence behind the boat tail, and begin to yaw, and then tumble. I can load certain boat tail bullets for subs just as good as flat base. But a flat base bullet is a shorter bullet, and is easier to stabilize.