Several embodiments of ammunition supply indicating systems for firearms are disclosed. Each system (1) counts, either automatically or manually, the number of rounds in a magazine introduced into the firearm, (2) detects the movement of a reciprocally movable slide in the firearm to increment a count of "one" for the round entering the firing chamber, and (3) electronically combines the two digits, and (4) displays same on a display panel. The display panel is situated in a thin housing secured to the firearm along the line of sight of the user. A running total of the number of rounds remaining in the firearm is visible to the user. Slide movement may be detected by magnetic, mechanical, optical, or electrical devices, while the number of rounds remaining in the magazine may be monitored by position switches operated by the follower within the magazine. A flashing question mark symbol may be visible on the display panel to alert the user that the magazine has not been inserted into the firearm and/or that a round may be present in the firing chamber.