Go onto a two part level. Then on the first level, grab a sword and use it until it is empty. Instead of throwing it away, keep it and use your other weapon. Then, after the second level starts, you will still have the handle. Even though you get the message saying "out of ammo", the game will mistake it for a sword and this glitched sword will have unlimited ammo.
Also if your sword runs out of ammo in normal playing, you will get the brass knuckle. It is just a very fast meleeing weapon that does the same damage as a melee from a plasma pistol.
Another way is to simply hit the melee button instead of the "fire" button.