As at 2020, why don't fighter jets have ≥ 200 rounds for their cannons? I already read Why do fighter jets still have guns/cannons?, but it doesn't expound why 100-200 rounds suffice or how &quot;6 seconds is actually a lot of 'gun time' for modern air-to-air systems considering speeds, targeting systems and lethality of the projectiles involved.&quot;

I acknowledge that &quot;It's not like WWII anymore where you'd slowly pull up behind a bomber and then have to spend several seconds pumping a bunch of crappy .50-cal lead in, hoping to poke enough holes in an oil cooler to make a difference.&quot;
Revolver cannons
&quot;The Mauser BK27 is a 27mm calibre [revolver] cannon manufactured by Mauser (now part of Rheinmetall) of Germany. It was developed in the late 1960’s for the Multi Role Combat Aircraft programme that ultimately became the Panavia Tornado, it is used on a variety of aircraft including Typhoon, which carries 150 rounds.&quot;
&quot;For that role the [SAAB JAS-39 Gripen] aircraft is armed with a 27 mm Mauser BK-27 revolver cannon with 120 rounds and with up to six short-range AIM-9 Sidewinder air-to-air missiles.&quot;
Rotary cannons
&quot;The F-35A is armed with a 25 mm GAU-22/A rotary cannon mounted internally near the left wing root with 182 rounds carried&quot;. The Dassault Rafale fighter
&quot;is equipped with the GIAT 30-millimeter cannon with 125 rounds of ammunition.&quot;
MiG-35 &quot;is also armed with the 30 mm Gryazev-Shipunov GSh-30-1 autocannon with 150 rounds.&quot;. &quot;The Su-35's armament includes a GSh-30-1 30mm autocannon with 150 rounds&quot;.
They don't need more rounds.

Modern fighters contain a targeting system that can calculate where the bullets will go, allowing the pilot to fire only when he's sure to hit the target. In some aircraft, the gun is slaved to the targeting system so it can be set to fire automatically when the target crosses the firing line.

In WW2, aiming was more primitive and usually involved firing a long salvo, using the tracer rounds to see where the bullets go and adjusting your aim to hit the target. This required far more rounds for a kill.

The gun is a weapon of last resort, to be used when all primary weapons have been expended. It doesn't make sense to add a large gun magazine when that would reduce the primary payload.

Gun strafing has been superseded by cluster bombs. Since the 2008 treaty forbade their use, small smart munitions have been used instead.