Shoot with crowbar: 

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.