I don't want to get into guns, that's a special case in the US due to money in politics.

But, ok, yeah "jet fuel" I was thinking more of military jet fuels, I was in the Navy for a while and never in the aviation industry. Though even they don't approach the energy density and instability a reactor would have. Maybe I should have said "rocket fuel" or even pure hydrogen. Anyway, point is cars could use fuels that are more energy dense, but even missiles don't use the most energy dense fuel available because there's a need to balance stability and potential for explosions with energy density.

A miniaturized fission reactor like I mentioned or even more, a miniaturized fusion reactor like in Ironman, would have problems if sold unregulated, both in accidents as well as malicious use as a weapon.