The FBI is under no bullet design restrictions, and chose a barrier blind expanding load.

Barrier blind gives you more than the haters care to admit. For example, bullets that do well vs the windshield test do well going through bone. Intermediate barriers could end up being the arm of your attacker.

Expanding bullets are more consistent in performance. Fragmenting designs can sometimes fail to fragment, or fragment too much without enough penetration. I'm more open to fragmenting designs in a larger caliber, the 5.56 just starts without much room to loose any bullet weight and go deep enough.