From what I've seen in some IL-2 BoS videos they did it somewhat correctly.

The ammo counter is a fairly simple device featuring a magnetic coil turning around a cylinder once the trigger is pressed and electric current is flowing.

[img]https://i.imgur.com/yL1MNZS.jpg[/img]

By dialing the knob on the front panel you'd be able to reset the ammo counter or set it to any position desired (done after servicing
and refilling the plane's ammunition supply for instance).

On the SZ 100 in particular this was done in a specific way. The SZ ammo counter was available as the SZ 100, SZ 500 and SZ 100 with
each number representing the rounds available. Each with a different scale and gearing to accomodate the different ammo count.

But neither of the SZ 100, SZ 500 or SZ 1000 would be a fit for the very common 200 rounds on the Bf 109's nose cannon.
In this case the SZ 100 was installed and simply did 2 full turns in order to represent the ammo count of 200.

Now in case of having 120 rounds the SZ 100 would be slightly "overturned" to accomodate for the 20 additional rounds.
It would then look like this (the middle one):

[img]http://i.imgur.com/sgSpbn1.jpg[/img]

After depleting those 20 rounds and turning accordingly the ammo counter would then just show the full white bar representing
100 rounds.


Hope that answers your question!

Regards, Reuter.