An auto-loading firearm having a system for switching between live fire and training modes via a selector switch mounted either on the firearm or on a specially designed training attachment. An action arm is selectively movable in response to movement of the selector switch between a live fire position, in which the firearm is free to operate as normal, and a training mode position where the action arm interrupts operation of the hammer and prevents the firearm from discharging while allowing the trigger and, if present, trigger spring to continue operating.