[this is wandering off the subject of this list, but...]
Lastly, an ordinary A-bomb is just a way to bring together a critical mass of a fissionable material, like U-235 or Plutonium. Once a critical mass is in one place the chain reaction will handle the rest.
No, not exactly. See Robert Serber's "The Los Alamos Primer/The First Lectures on How To Build An Atomic Bomb" (ISBN 0-520-07576-5), published by the University of California Press. I'll quote from the beginning of Chapter 18: "To avoid predetonation, we must make sure that there is only a small probability of a neutron appearing while the pieces of the bomb are being put together. On the other hand, when the pieces reach their best position we want to be very sure that a neutron starts the reaction before the pieces have a chance to separate or break. It may be possible to make the projectile seat and stay in the desired position. Failing in this, or in any event as extra insurance, another possibility is to provide a strong neutron source which becomes active as soon as the pieces come into position. ... Note that this is especially important with the more efficient implosion-type bombs, where the critical mass just can't "seat" like on one of the simple gun-type bombs. Also note that you can't build a gun-type bomb using P239; it reacts too quickly. "Evidently a source of this strength that can be activated within about 10**-5 sec and is mechanically rugged enough to stand the shocks involved with firing presents a difficult problem." The text later states that the initiator used in the first two bomb designs was a mixture of Polonium and Beryllium. - Bill