Is that actual mass, or TNT equivalent? I'm wondering if it would be possible to have a nuclear weapon small enough to fit inside a bullet. (Probably not...)
Not with U235 or Pu239. The minimum theoretical mass (without crazy implosion schemes or the like, which also aren't going to fit in a bullet) is a few kilos.In a bullet, no. However, there have been nuclear artillery shells. The Davy Crockett round is about near the lower limit of size for fission based weapons.
Davy Crockett was 11" dia, as were the smallest nuke artilerry shells that I know of. (11" = ~28cm, if anyone wants real units...)