Garibaldi kills the alien by putting 6 .38 special rounds into a pipe, and then inserting the pipe into a larger steam pipe, using the heat to set off the rounds. Firstly, steam isn't hot enough to set off "modern" gunpowder ammunition. Secondly, even assuming it was hot enough to set them off, they were all in the same pipe, in a line. The moment one went off, it would either blow the rest of the unfired rounds out of the pipe, or the pipe itself would explode open. They would not go off, one at a time, in the proper order, as shown. Furthermore, 8 rounds fired out of the pipe, instead of the 6 that he put in.
At the beginning of the episode, Zack enters the security office while Garibaldi is admiring his grandmother's vintage Smith and Wesson .38 caliber revolver. He spins the open cylinder before flipping it closed. The clicking sound heard as the cylinder is spinning is not possible in a revolver of that type.