Some of you might wonder why do guns (especially in combat airplane games like war thunder) looks like it fires slower than mentioned? (Like when you play war thunder and you fire the guns there is like 5 bullets per second instead of the mentioned 10 bullets per second)
Well, it's because something called tracers. They are used to; well, trace the bullets to show where it is and where it's going.
Different bullets have different uses for combat and have different advantages and disadvantages, like:

Tracer bullets: to trace bullets lol
AP/Armor piercing: to pierce tougher armor
Incendiary: to cause explosion and heat on impact
Universal/Omni-purpose: a kind of mix of everything

So this solves the question: why does it seem like less bullets are being fired?
It is because not all bullets in the magazine/belt have tracers so not all bullets are visible.
However, bullets with no tracers can be used for stealth attacks or surprise attacks.