To use the crowbar to shoot bullets from the chaingun or flame from a pyro, do the following. Change your class to an HW or Pyro, then use up all your ammo from your chaingun or flamethrower. After wasting all the ammo, get out the crowbar and hold the Right Mouse Button (or whatever displays your special ability menu) and run over a pack of ammo. You can then shoot with your crowbar.
Note: This must be done on the server machine. Type "sv_cheats 1" at the console, then change the map with the "changelevel [map name]" console command. Finally, type "tf_weapon_[weapon name]" at the console on any machine connected to the server to select that weapon.
Play a multi-player game and get the heavy weapons character. Have your Gatlin Gun ready and get ready to switch your gun. Have the gun that you want ready, then hold the LEFT MOUSE BUTTON for three seconds. You will hear the gun rotate. The gun you selected will have the same ammo that the other gun. This also gives you an extra 192 shots with the shotguns.
Change your class to a scout, go in the console, type "+reload", and go underwater in any level. Go in the console and type "kill". Wait for someone to get your pack, then you will be a zombie. To respawn again, go in the console and type "-reload".