If you cant melt objects IN the microwave, its very doubtful that you will be able to melt them at a distance. You cant concentrate the waves into a small beam like a laser either.
What you CAN do is fry electronics with it, all the copper tracks act like antennas and get hundreds of volts across them, literally frying them. If your microwave is still intact, try sticking a circuitboard in there, but not for long, it will smoke like crazy. Also, try putting a lightbulb in it, its pretty cool.
As for making a gun out of it, i wouldnt encourage it because you can blind yourself, make yourself sterile . . . Not to mention blind the unsuspecting person who you didn't realize is standing right on the other side of the wall, which is invisible to microwaves :cry: . And youl slap yourself across the face when you accidently channel microwaves right at your computer . . . :lol: