FMJ bullets tumble after striking the first wall and then fragment at the cannelure. Other bullets do the same but fragment because the jacket is thinner
I use M193 55gr FMJ for HD.
Not inherently. The military spent quite a bit of time and money trying to get their 5.56 tumbling or 'yawing' in flesh. The only reason they did this was because FMJ bullets were designed to penetrate, and did very little damage in tissue, for this reason. The military realized that sometimes the long bullets would yaw in flesh and this would do more damage than the bullet penciling straight through, without violating any of the conventions that militaries are bound to, of not using expanding bullets. For a SD situation, you are not bound to the same laws as the military, and using expanding rounds is preferable, unless you are looking for barrier/cover penetration. They generally do more damage, penetrate less, and transfer a larger percentage of energy into the target, and they do it consistently.