| Wild guess:  Shotguns are generally less expensive and less regulated than pistols.  In most jurisdictions, I can pay cash and carry a shotgun out the door, but not a pistol.  Thus, if I wanted concealable firepower quickly, I can buy a $300 shot gun, illegally saw it down, and use it to rob my local 7-11 (All in the same day! =D).
 (Note, this is only hypothetical...)
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