The Chechens are a supreme example of this in regards self made guns.
Those are some damn rugged, cool looking implements of death. Also look a lot like the Orks guns in 40k, which only adds to the cool factor. I particularly like the .50 cal that looks like it's made from, and could kill, a truck. The Russians are no slouches either
Not only can it drive underwater, it can run off bloody anything. Amazing stuff, huge props to the guy who made it.
The examples don't have to be related to war, but most of the cooler ones are.