| Let me echo: This will not work. You will end up with a loud bang, some shrapnel, probably a slightly deformed car, and a variable amount of atmospheric contamination. Depending on whether you used U-235 or Pu, you will also most probably be dead of either radiation poisoning, heavy metal toxicity or (better yet!) both.
There is a critical component missing from the list above, which is an incredibly oversimplified description of a gun-mechanism atomic bomb, such as was dropped on Hiroshima. While it's true that this type of design is easier to engineer (its design wasn't tested during the Manhattan Project because they were so sure it would work) high 'first shot reliability' is achievable only by using extreme overkill in both the design and construction of the weapon - especially in the amount of fissile material. This would jump your requirements for fissile material appreciably; while it is true that critical mass is only 16kg or so, remember that you're not going to be able to get and keep the whole mass in critical proximity and keep it there long enough to get an explosion unless you have an insanely efficient design.
If the design is not efficient enough, you will get an explosion, of sorts - you'll get a fizzle, which is where the fissile material heats itself up quickly enough to burst apart from thermal stress before reaching supercritical and hence detonation. Don't get me wrong, this is still a Bad Thing - but it isn't a big, big bang. Remember: in order to work, your design has to accept the driven mass of material being slammed into the rest mass at extremely high velocity without breaking apart or bursting or even rebounding for long enough for the Good Stuff to happen.
One of the ways they get it to do that is to use an initiator, which is the missing piece. An initiator is simply a disc of special metal (you can find out what works if you care) placed between the masses. Said metal dramatically increases the neutron count when the fissile masses get near to it. This has the effect of 'lengthening' the time window that the bits of fissile material are in 'critical proximity' to each other , which in turn eases many of the engineering challenges somewhat.
While Little Boy was in fact unstable when armed, this was mostly because the conventional explosives in it were primed in multiple, redundant and not-very-safe ways in order to ENSURE that enough of them worked when called upon to produce the proper form of detonation. Had the aircraft crashed, what was likely to happen was that the conventional systems aboard the aircraft would have been damaged, and it is quite likely that the conventional explosives would have detonated as they were over-rigged to do.
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