Wikipedia article on gun-type fission bombs..

The simple part of the gun mechanism is pushing two sub-critical lumps of U235 together to make a large enough mass to be super-critical. You could do this with your hands pushing the two lumps together and it still happens, the bullet design just makes it more efficient.

The speed of the bullet is because the super-critical chain reactions take 1 microsecond. That means you have only 1 microsecond to get the bullet to mix with the target and hopefully form something like a sphere. If the bullet is moving too slow, you get pre-detonation.

Basically, bullet too slow = the bottom blows up and wastes the remaining unreacted mass of U235 by throwing it out the top.

The bullet was hollow, but more similar to when you do the hand gesture of penis entering vagina. It kept the two lumps of material from touching for as long as possible. The hollow bullet landed on a spiked target.