Conventional self loading firearms (automatics) operate either by tapping off gasses from a fired cartridge, recoil from the round firing, or having the fired case blown back from the gasses of a fired round.
In all of them, a cartridge must fire to power the weapon into reloading itself. Of course, bolt, pump, and lever action firearms are operated by human muscle power.
A dummy cartridge, or snap cap, does not fire, so produces none of the energy needed to cycle the firearm.
The only exceptions that would come to mind are rather exotic externally powered gatling guns or cannon, such as the Hughes 25mm Chain Gun in the Bradley Infantry Fighting Vehicle.
What we have here is... failure- to communicate.