Use the following steps to use the crowbar to shoot bullets from the chaingun or flame from a pyro. 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 should now be able to shoot with your crowbar.