There may come a time where you'll need to make your own small, cheap, concealable firearm.
And that's all I'll say on the subject.
Here's something pretty neat. A full-auto (Assault Weapon for you hoplophobes) .22 LR machine pistol. Test fire at the end of the video.
12 Gauge single shot:
There are more videos on the interwebs if you're so inclined.
For more ideas, pick up a copy of TM 31-210. I picked up one at my local Army Navy Surplus store for $5.