Really curious USA do error with F-4 without gun fixed later with a gun pod super sophisticated F-22 have one even in futur combats with normaly a short game less important remains interesting as last weapons and if the driver runs out of missiles ... so little disadvantage for J-20
Okay forbin, the F4 Phantom II lack of a gun as leading to the kill rate is not a controversy it's a contrivance. In actual fact there were three US services flying F4 Phantom II in the Vietnam war. The USN and USMC both flew gun less F4, the USAF flew a version of the F4 with a internal 20mm M61 Vulcan cannon. And throughout the war it was the USAF with the poorest Kill ratio. So what went wrong?
First the F4 was designed as an Interceptor not an air superiority fighter.
Second as I said the Gun equipped F4 of the USAF had the poorest Kill ratio. When the kill ratio improved for the phantom late war it was the USN and USMC F4 that got better, and even then it was not because of a Winchester, the vast majority of kills were with missiles.
What happened? Well it was a gun, TOP GUN the navy started taking more time to train there Aviators and maintenance crews.
Yes J-20 without gun is a big surprise for me as well. Perhaps they will add it one day and there is enough of a space for future application with higher thrust engines. In the meantime, J-20 without a gun shows PLAAF is supremely confident of its BVR abilities and probably do not plan to ever use such an expensive asset as anything but a scout, shoot, and run type of fighter. I doubt VLO fighters are that effective at evasion. Why would the Americans put guns on both F-35 and F-22. J-20 needs a gun even more considering its current missile load is smaller than F-22.
A number of fighters still lack guns they are typically grouped into fighter bombers or interceptor.
Also F35 like the F4 Phantom II before it comes in flavors for the buyers. The F35A of the USAF has a gun. The F35B SVTOL version and the Carrier F35C use a gun pod. The Air to Air missile is the system of choice and the weapon that makes the vast majority of air to air kills.