It is, apparently, Cf-252 that has the ridiculously low critical mass, on the order of a few grams, not Cf-251.
The trouble with the concept, as well as cost, is decay. The fissile material has a half-life of just 2.6 years, so you can't stockpile them. And the decay heat would gradually cook off the propellant in the bullet, so the magazine would need cooling. Oh, and Cf-252 has a higher than ideal spontaneous fission rate for nuclear weapons.
Cf-252 would probably stretch to a nuclear mortar shell or hand grenade, though. And with a longer half-life (900 years) it would probably remain effective long enough to get from the plant to the battlefield.
There's an article here claiming that the USSR tested a californium bullet, but decided against employment. Pravda, so make of it what you will. Some discussion of the physics here.
I really can't see the practical applications of such a weapon, though. Maybe special operations forces - I can see dropping a few hundred tons on theatre nuclear weapons being useful - but production, stockpiling and cost would be major issues. They'd pretty much have to be built for a specific job, given the rate of decay.
The trouble with the concept, as well as cost, is decay. The fissile material has a half-life of just 2.6 years, so you can't stockpile them. And the decay heat would gradually cook off the propellant in the bullet, so the magazine would need cooling. Oh, and Cf-252 has a higher than ideal spontaneous fission rate for nuclear weapons.
Cf-252 would probably stretch to a nuclear mortar shell or hand grenade, though. And with a longer half-life (900 years) it would probably remain effective long enough to get from the plant to the battlefield.
There's an article here claiming that the USSR tested a californium bullet, but decided against employment. Pravda, so make of it what you will. Some discussion of the physics here.
I really can't see the practical applications of such a weapon, though. Maybe special operations forces - I can see dropping a few hundred tons on theatre nuclear weapons being useful - but production, stockpiling and cost would be major issues. They'd pretty much have to be built for a specific job, given the rate of decay.