Speaking of building your own "garage-expedient" firearm, this may just be a trailer for a movie, and thus just a dramatization, but, yes, making a firearm-like device capable of grievously wounding / killing someone really is this simple:

Look at the tools and materials she used: wire snips, a block of wood, a knife, a short length of steel pipe, a file, the pipe end cap, copper wire, a small square of cotton [...]