yeah, from what I understand if you have uranium that is enriched enough you could simply take two chunks and smash them together really hard with your hands and they'd go off.
Actually this has happened before. Look up the "Demon Core;" it was a core of plutonium that was involved in two accidents here in the U.S. and it poisoned several people. The first time, a guy was playing around with tungsten carbide reflectors and he moved them too close, causing it to go critical. The second time, another scientist was playing around with reflectors, his hand slipped and it went supercritical again, and Darwin'd the guy who made the mistake.
In bomb, however, it's much, MUCH more difficult. The process doesn't have to simply cause the core of the weapon to go supercritical, it has to sustain supercriticality. Since the reactions produce heat, and therefore expansion, there has to be a force opposing the expansion until the levels of energy released are sufficient to cause the weapon to produce catastrophic damage.
While the gun-type weapon is fairly simple, you need a lot of fissile material and the process is very inefficient. "Little Boy" was a gun-type and the gun-type was never officially tested before they dropped it; theoretically, it was a simple design. "Trinity" was the first implosion-type nuclear device tested; implosion weapons are far more complex than the gun-type and require immensely precise design, machining, and timing to even work.
