Towards the end of the movie, Mary Lincoln shoots the main female vampire. She uses a Sharps carbine to do this. Prior to shooting, Mary works the lever--a common motion seen with lever-action rifles to load a round from the magazine into the chamber for firing. Problem is, even though the Sharps carbine is a lever-action, it is still a single-shot rifle, using a paper cartridge. In order to operate the firearm, you push the lever forward, opening up the rear breach; then slide a paper cartridge (complete will ball or bullet); you then pull the lever back into place, which closes the breach & tears off the rear of the paper cartridge so that the flame of the percussion cap can set off the charge.

If the gun was not already loaded & primed, then Mary's cycling of the lever would have done nothing, so the gun would not fire. If the gun was already loaded, then opening the breach would have caused the gunpowder to fall out of the open breach, resultining in the gun not firing, misfiring, or gumming of the breach so it wouldn't close. And at no time did she place a percussion cap on the nipple, which means the gun would not fire.