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The cool thing about californium is that it emits neutrons much more profusely than do most radioactive atoms. As a result, its critical-mass requirement is quite small -- a few grams. Herman Kahn's book On Thermonuclear War describes a hypothetical nuclear weapon utilizing californium fuel: an "atomic six-gun" the size of a regular revolver shooting hollow-point californium-tipped bullets. Upon impact, the kinetic energy of the bullet crushes the hollow-point tip into a critical mass and boom -- a nuclear explosion in the sub-kiloton range, one per bullet.

This sounds interesting but I must admit I don't see how it could work. We're talking about spontaneous fission with 252Cf, and a lot of it. The problems with predetonation become quite difficult to manage with this kind of material. Add to that the fact that in any kind of impact the compression will be quite asymmetric and difficult to control, which makes it difficult to attain a supercritical assembly, much less maintain it for the time necessary to achieve a significant fission rate. If Kahn suggested something like this, either he was free-associating ideas or knows a lot more about the physics of the weapons than I do (which is probably true).