For a real one I think I would say the Mateba 6 Auto Revolver in 44 mag or 454 casull
Modern Firearms - Mateba Model 6 Unica
Mateba Autorevolver - Wikipedia, the free encyclopedia
For a fake one I would say any updated version of a "phaser" with disintegration capabilities (no body no evidence).
You just have to love the double whammy, two bad guys with one shot, priceless.The Law Giver (Judge Dredd)