Some fighters were fitted with ammunition counters, mostly Axis aircraft, e.g: "One feature of the 109’s cockpit that found much favour with its pilots was an ammunition counter, something not incorporated in contemporary Allied fighters" (Messerschmitt Bf 109 A-D Series, Robert Jackson); "Like German fighters the Italians installed round-counters on the [Fiat CR.42] cockpit instrument panel" (Fighting Cockpits, Donald Nijboer). In this Smithsonian photo of a Focke-Wulf Fw-190 cockpit the counters are the four white bars at the top of the instrument panel.

Few Allied aircraft had such, though they weren't entirely unknown (e.g. the Grumman F6F had one), and they were certainly suggested. Jeffrey Quill, Supermarine test pilot, briefly served with an active squadron to gain combat experience and wrote of the outcome in A Test Pilot's Story:

"I also wanted round-counters. You became involved in a fight and blasted off in a few bursts of fire. Then it was over for the moment and you reformed with some, if not all, of the squadron. Everybody's fuel state was immediately available because both tanks of the Spitfire were accurately gauged. But how much ammunition was left? Nobody knew, except those who had not fired at all. This meant that pilots tended to assume the worst and return to base to re-arm when in fact further action was probably quite feasible. I felt, therefore, that a round-counter would be of great value and not beyond the wit of man. (This was never done.)"

As Quill said, it wasn't beyond the wit of man; Jon E. C. Tan's Aces, Airmen and The Biggin Hill Wing includes sections of LAC Raymond Davies' memoirs recounting the trial installation of a cannon round counter in Bob Stanford-Tuck's Spitfire: "Tuck was so excited and impressed by the system we rigged up to count the rounds being fired from the drums. He said it was just what we needed and that he was going to contact Group HQ and the Air Ministry to tell them of our invention and recommend that it be adopted as a proper modification for all cannon-armed fighters."

Tuck was shot down a week later, so may never have had a chance to make his recommendation, or it may not have been considered a priority. There were methods of indicating low ammunition: "Some fighter pilots loaded the last section of their belts with tracer, to give them a warning that they were close to running out of ammunition" (Flying Guns - World War 2, Tony Williams); the P-51D Mustang carried more ammunition for its inboard guns than the centre and outboard, so when the outboard guns stopped firing the pilot knew he was down to about 100 rounds for the inboard guns. By and large, though, in the absence of ammunition counters the pilot didn't know:

"Anyway, I wonder how much ammunition I've got left. I don't expect there is all that amount." (First Light, Geoff Wellum)