The above will not work.

If you attempt to make it work, you may end up with a bad case of cancer (or otherwise in a bad way), or you may just look really stupid, or both.

It will work slightly better if you use enough reactant (the smallest theoretical critical mass for a bare sphere of metal, under ideal conditions, is somewhere around 16 kg for weapons-grade uranium or 11 kg for elemental plutonium-- but you'll need a lot more than the minimum for a gun-type weapon as described above). It still won't work very well, though; you certainly won't get a mushroom cloud by any means. This is because the main problem with building nukes is not bringing the material to critical mass, but rather keeping the chain reaction going long enough that a significant amount of energy is released; for that, you need either a different design or a lot more reactant, plus a lot of math and very precise machinery. More than many small governments can get their hands on, let alone a lone malcontent.

If, by some fluke, you actually manage to make a working fission bomb with a significant explosive yield by the above method, be absolutely sure you set the timer for a long period of time. An amateur fission bomb made with the minimum amount of reactant certainly won't destroy dozens of square miles, but it will still be hard to get away from if you set the timer for, for example, thirty seconds.