I know you were thinking handgun, but check out the Ruger 10/22 rifle. Cheap ammo, great gun, and you can shoot hi capacity magazines as fast as you can pull the trigger. They have lots of mods available. If you want it compact, you might be able to modify it with a folding stock, so it breaks down pretty small. People have even (probalby not legally) modified them into machine guns. Google "10/22 full auto" for some cool redneck You Tube vids.
The 10/22 is also cheap. Like under $200 at Sports Authority. Check it out.
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